In the episode "Return to Tomorrow", Sargon explains that his civilization destroyed itself when it became so advanced and arrogant that "we dared think of ourselves as gods".
The Sufficiently Advanced Alien Q plays with this in Star Trek: The Next Generation. In "Tapestry", Picard dies and enters the "afterlife", where he finds Q awaiting him, who informs him that he's dead and that Q himself is God. Picard rejects this, because he doesn't think that "the Universe is so badly designed". Q just snarks that Picard is lucky Q doesn't smite him for his blasphemy.
Jason Ironheart in the episode "Mind War". Unlike most of this trope, he conducts himself with responsibility, humility, and some amount of restraint (he kills someone while escaping, arguably to save more lives).
Emperor Cartagia, who was convinced (if he needed any convincing) by the Shadows that he was a god and Centauri Prime should be sacrificed to him.
Note that it was accepted in Centauri religion that Centauri Emperors become gods after they die. Cartagia's peculiarity was in believing that he was a god while he was alive.
Illyria also did this... she is a powerful Old One, but not really a god like Glory. She states it almost word for word, though. "I am a god!"
At one point she claims she was once "god to a god" and thus might be more godlike and powerful than Glory. It's hard to tell though since both of them were stuck in human forms and their powers were extremely diminished (Illyria has far better feats and range of powers, enough that it was killing her, but that might just be the differences in how they ended up in their respective positions).
Willow, Giles, and Xander combine their essences with Buffy through a magical ritual, giving her the power to defeat Adam in the penultimate episode of season 4.
Willow becomes this trope several times through the series, in a good way in season 5, in a bad way in season 6, and good again in the final episode of season 7
The First Evil, posing as Warren, convinces Andrew that they will all live as gods (complete with Andrew dreaming of them all dancing about in a field of flowers, wearing togas, and playing a lyre, while singing "We are as gods!") All Andrew has to do to bring this about is kill his only friend, Jonathan.
The Goa'uld not only fit this trope perfectly, but are also really extreme cases. Although they are just highly advanced aliens, they do pose as god in front of their primitive slaves. Many — like Ra, Cronus, and Apophis — seem to buy into their own propaganda and think they really are gods, though Ba'al is aware he is just a parasitic snake with a host and Yu never proclaimed himself a god, instead setting himself up as a legendary emperor. Both Ba'al and Yu have disdain for those who buy into their delusions (although Yu grows generally delusional at a certain point on account of his senility), and Ba'al manages to outlive all the others and obtain the greatest degree of power. Anubis is... ambiguous, though out of all the Goa'uld, he has the best case for calling himself one—being that unlike other Goa'uld, he Ascended to a Higher Plane of Existence, and, despite being rejected by the other Ascended beings, remained a half-Ascended Energy Being. The Goa'uld in general border on a deconstruction, as this quite frequently leads to their doom.
The Ori take this far enough that it becomes ridiculous when people insist that they're not gods, since, well, everything a god can do the Ori can do. (Including Virgin Birth) If that's not a god, one has to wonder what people would accept as a god. Teal'c later suggests that acts, rather than power, make gods. Or at least, that being a god is not the same as being worthy of worship. They couldn't stop the team from killing them. Adria explicitly states in The Ark of Truth that the Ark can only make people believe what is true, which is why she can't use it. Since the Ark makes the Priors believe that the Ori are not gods, clearly the Ori are actually not gods. Although she fully believes she could use it if it weren't for the existence of Ancients undermining her by being just as high up in the grand scheme.
The Outer Limits, "The Sixth Finger," combines both versions. An illiterate miner is put through a process that quickly evolves him, resulting in increased intelligence and psychokinetic power, which he unleashes on the pathetic lesser beings around him; but then he evolves even further and rises above such petty emotions, realizing the true nobility of existence.
I, Claudius has a variant mirroring real life when Caligula declares himself to be a god; however, in Caligula's case there were no Green Rocks or Vorlons — his declaration is caused solely because he is, at that point, absolutely barking mad. Some Roman emperors were deified after death — the last words of Vespasian were, "Alas, I am becoming a god" — so Caligula was jumping the gun a bit. (Deification even after death was in real life somewhat rare: among others, Tiberius and Nero were not deified after death - and neither was Caligula. His pre-death self-deification was also quietly shelved in Claudius's time.) "And his sister's become a goddess. Any questions?"
Heroes: A number of characters, most emphatically Adam Monroe. Meanwhile Peter Petrelli, who has a claim to godlike powers since he absorbs the powers of everyone he encounters, has yet to fall victim to this.
If we can assume that Maury Parkman's illusion of Linderman accurately portrays the actual character's personality, then we can infer that Linderman thought of himself as either a god or very close to one. In one season 3 episode, he implied that he spoke the word of God.
However, he may have been simply trying to convince Nathan, who had recently gone through a bit of a religious awakening, that he was God in order to manipulate him into doing what his father wanted him to do.
Doctor Who: In the new series, the Dalek Emperor had a pretty high opinion of itself. So much so that insulting it would trigger the now-famous line, "those words are blasphemy!" and a chorus of "Do not blaspheme! Do not blaspheme!" from his subordinate Daleks.
Also, in the same episode, Rose Tyler absorbs the heart of the TARDIS and is turned into a godlike hybrid called Bad Wolf, who disproves the Dalek Emperor's "I am God" hypothesis by disintegrating him.
The Doctor: This is wrong. You can't control life and death!
Rose: But I can.
Played with in the original series serial "The Armageddon Factor", when the Doctor, possessing the full power of the Key To Time, starts making an A God Am I speech, much to Romana's alarm. He's just kidding/making a point about how dangerous the power is, in anyone's hands.
In "Genesis of the Daleks," Omnicidal Maniac Davros is asked, if he had in his possession a virus that would wipe out all life, would he release it? His response:
"Yes... yes. To hold in my hand a capsule that contained such power... to know that life and death on such a scale was my choice. To know that the tiny pressure on my thumb, enough to break the glass, would end everything. Yes - I would do it. That power would set me up above the gods! And through the Daleks, I shall have that power!"
The 1982 story 'Four to Doomsday' focuses an alien who not only believes himself to be a god but, in fact, destroyed his home planet in an attempt to travel back in time to the creation of the universe to see how he had created it.
In 'The Waters of Mars', the Doctor snaps after being pushed to the limit and declares himself to be, in essence, a god. And he is not under any sort of mind control—nor can he be said to be having delusions of grandeur, since his claim to godlike power is entirely accurate. Rather, he's giving us a glimpse of what the last of the Time Lords could become without the restraint that his Companions give him.
Of course, having said that, he is promptly snapped back after Brooke commits suicide rather than muck around with her destiny, so it worked out in the end. Sort of.
In an episode of Law & Order: Special Victims Unit, a religious cult leader is on the run with one of the children from his cult, a 12-year-old girl who is pregnant with his baby. Olivia has her gun on him, and the girl has a gun on Olivia, when the leader starts telling the girl that the police want to destroy him because "They know that I am greater than man, I am greater than God!" The girl promptly shoots him, and sobs to Olivia that she did it because "He said he was greater than God... but nobody is."
Also in the original series a jerkass doctor is investigated and he says in a meeting "God doesn't make babies I do.
Kilobyte the Freaky Tentacles Guy (yeah, Squick please) in Ace Lightning... though since he's a videogame character, he was kinda trippin...
Callisto in Xena: Warrior Princess consumes ambrosia, the food of the gods, to become a god during a battle with a vengeful Amazon that had also consumed ambrosia solely so that she could kill Gabrielle. Uniquely, in later episodes she regrets her godhood and tries to find ways of undoing it, culminating in her begging Xena to find a way to kill her. She changes her mind after witnessing Gabrielle make a Heroic Sacrifice - unfortunately for her, Xena had already found a way to kill her, and didn't particularly appreciate her gloating about this.
In the Season 1 finale of Dollhouse, Alpha proclaims that he has "ascended" because he contains multiple personalities and gives this "gift" to Echo who rejects it.
Battlestar Galactica ("Razor"). The old man hybrid states that his Centurian guardians believe him to be a god, and he doesn't dispute the claim. He certainly seems to be all-knowing, but as the protagonist proves, not immortal. Provided that all this doesn't happen again, and again, and again, and again...
Caprica: Given her godlike powers in V-World, Zoe proclaims herself God when confronting Clarice Willow in the Apotheosis simulation in the series finale.
In The Twilight Zone episode "The Little People", a pair of space-farers found themselves stranded on an apparently desolate planet. One of these men found a civilization of microscopic people, and quickly set himself up as their god. Though his fellow tried to talk him out of it, the newly deified pilot decided to stay as the god of these tiny people. This being the Twilight Zone, things went south quickly for him.
Craig's boast, "I'M THE GOD! I'M THE GOD!" became a catchphrase on MST 3 K.
From the episode "He's Alive", after Volmer kills his childhood neighbor Ernst, his mentor aka Adolf Hitler asks him how it felt:
In the Coupling episode "Faithless", Jane learns that James, who she's attracted to, hosts a religion program. In the course of Digging Herself Deeper, she comes out with:
Jane: You see, I find religion very relevant to my work. When you spend your days in a helicopter dicing with actual death, sometimes you look down at the ground so far below you, and you find yourself thinking, "I'm a bit like God." James: Okay. Jane: Only without the omnipotence and the X-ray vision. James I... don't think God's the one with the X-ray vision.
Parodied on Bottom; Richard Richard is every so often given to convincing himself that he's the Second Coming (or on one notable occasion the Mother of God, which in his tiny mind amounted to the same thing), citing his own virginity as the reason that he's been kept 'pure', because he's 'better than everyone else in the world'. The more likely explanation behind said virginity is that he's merely a repugnant, violent, pathetic, sweaty pervert with ludicrously pretentious delusions of grandeur, among them his tendency to convince himself he's the Son of God.
Castiel as of the season 6 finale, wherein he absorbs all the souls of Purgatory into himself. He proceeds to do to Raphael exactlywhat Lucifer once did to him, shrug off a Back Stab with an angel-killing dagger, and declare himself the new Lord.
"I'm not an angel anymore. I'm your new God. A better one. So you will bow down and profess your love unto me, your Lord...or I shall destroy you."
When the brothers first meet Chuck, and prove to him that they are indeed the characters he has been writing about, his immediate reaction is this trope. The brothers didn't believe him, and he turns out to be a Prophet. At the end of the following season, it's implied that he actually IS God.
Chuck: Well, there's only one explanation. Obviously I'm a god.
Sam: You're not a god.
Chuck: How else do you explain it? I write things and then they come to life? Yeah, no, I'm deinitely a god. A cruel, cruel, capricious god. The things I've put you through ...
Eric Wolpaw, the creator of Supernatural, and Rob Benedict, the actor who portrayed Chuck, have confirmed that Chuck is indeed God.
Parodied in Community; the 'Clip Show' "Paradigms of Human Memory" reveals that Pierce at one point apparently thought he'd ascend to Godhood by... taking more than one flu shot.
George Marks from Cold Case, who justifies his string of murders by claiming to be God. When Lilly tears his claim to shreds by saying he's nothing more than a scared little boy, he flat out loses it.
Benjamin Cyrus in "Minimal Loss" saw himself more as God's messenger as opposed to actually seeing himself as God, but some of his actions made one think he actually was God. Furthermore, when he took over the cult from its old leader, Leo Kane, Cyrus told Kane that God said that Cyrus was going to take over the cult, and when Kane asked him when God was going to make the change, Cyrus pointed a gun to his head and said "He just did".
Billy Flynn in "The Longest Night" told his kidnapping victim, Ellie Spicer, that his serial killing prowess made him "like God".
Randy Slade, the school shooter from "Painless" had a serious god complex. He burst into the school cafeteria with a gun and a bomb and challenged his fellow students to "look God in the eye" and shot them if they didn't make eye contact with him.
Shooter: I am God. Now who is brave enough to look God in the eye?
House: Referenced in "Larger than Life", wherein Arlene Cuddy (Lisa's mother) calls Gregory House out for his behavior.
Arlene: In the clinic, you were a complete schmendrick. But once you knew I was Lisa's mother, you held your tongue. That's because you love her. I still think you're a pain in the ass with a God complex, and I'll kill you if you hurt her, but I'm glad she has you.
Sir Arthur Conan Doyle's The Lost World: In "Absolute Power", Challenger is transported to an alternate reality where he has tremendous powers. His arrogance increases tenfold along with his powers.