While several towers in the Bloons Tower Defense games can be considered this when fully upgraded, the Super Monkey stands out: It starts out with pretty good range and it has upgrades that give it much bigger range. It starts off firing darts at a decent rate, but it can swap those darts out for laser beams that pierce bloons and pop frozen bloons. It can also swap those laser beams out for plasma vision, which fires twice as fast and can pop lead bloons.
The fourth game gave introduced an upgrade known as the Sun God, which fires 3 sun beams at a rapid rate, effectively tripling the efficiency of the tower. The 5th game gave that an upgrade: The Temple of The Monkey God, which can sacrifice the towers around it and absorb their powers. Its power can vary from being weaker than a regular Sun God to being powerful enough to single handedly destroy a Z.O.M.G. in seconds.
The 5th game also introduced 2 upgrades after the epic range one. The 1st of those 2 is the Robo-Monkey, which has 2 arm cannons that can fire independently from each other, allowing the tower to target 2 different bloons at once. The 2nd of them is the aptly named Technological Terror, which fires its own special green plasma that is twice as powerful as the regular purple plasma while keeping the 2 arm cannons. It also has an activated ability where it sends out a short ranged shockwave that can destroy nearly any bloon in 1 shot.
There is also the Ray of Doom for the Dartling Gun, which shoots a constant solid beam of bloon destruction with a pop rate of 100 (formerly Infinite), practically killing any and all regular bloons in less then a second. The problem though, is that it is notoriously weak against MOAB-class bloons, since it only pops 1 per frame for those. Even so, a single one is still pretty effective against the M.O.A.B.s and B.F.B.s, but the Z.O.M.G.s are another story...
In the sense of money production, the Banana Farm. It can increase the amount of money per round that players get, while most of the upgrades increase its efficiency. Its bank upgrades actually stores it as interest until the player collects it. It can store up to $5000 money as a bank and $20000 money as an investments advisory.
The Cripple MOAB sniper is the perfect tower for stopping blimps. Its blimp stopping power depends on the strength of the blimps, but many later round strategies include entire armies of these.
In a similar sense, the Supa-Wide Funnel upgrade for the Bloonchipper. It makes the funnel huge enough to swallow blimps. It also has enough suction power to pull them into the funnel, where they get shredded for 500 damage.
The Cob Cannon, which can fire One-Hit Kill missles at any lane. It's most apparent during Survival Endless...
There's also the Cattail, which fires homing spikes that can hit any zombies on the screen and pop balloons, making Balloon Zombie elimination much easier.
The Gloom-Shroom fires piercing fumes at 360 degrees, allowing it to hit all zombies in its radius. However, it only hits one square in each direction. Its downgrade (Fume-Shroom) only fires at zombies that are in front of it, but can hit all zombies in the 4 squares that its shots cover.
Spikerock, which can take 9 smashes before dying (while all other plants can only take 1), and hits zombies twice when they are on top of it.
The Doom-Shroom, which causes an explosion that covers a large portion of the screen. Unfortunately, it leaves a crater on the spot where it just exploded, leaving that tile unplantable for 3 minutes.
The Winter Melon upgrade for the Melon-Pult. The Melon-Pult already does 4 peas worth of damage per projectile, but the Winter Melon brings it up an extra notch because its projectiles freeze the zombies on impact.
The Hypno-Shroom, depending on what zombie it is used on. The Dancing-Zombie is a great choice due to its ability to summon back-up dancers, while Football Zombies and Jack-In-The-Box zombies are other good targets to hypnotize.
Within a small degree, the Chomper. It takes a long time to chew the zombie that it just ate and goes down in 6 bites like nearly every other plant, but it recharges quickly and can swallow any zombie that isn't Gargantuar-sized, even if they have the same health as a Gargantuar.
The Coconut Cannon in the sequel is a pretty good unit. It behaves somewhat like the Cob Cannon, but when tapped, it fires a powerful coconut at the zombies.
Plants vs. Zombies 2: It's About Time also gave us the plant food powerup, which enchants the power of the plant that it is used on. For example, Peashooters fire rapid pea bursts, Walnuts gain an extra layer of health, and Potato Mine places 2 armed Potato Mines on 2 random tiles.
The Missiler Bots from Mini Robot Wars function similar to the Cattails from Plants vs. Zombies, but they cost only 50 energy, they can be placed on any platform and their missiles find another enemy if their current target is destroyed. They also have an upgrade that allows them to fire faster.
The Surveyor Bot can also be seen as this, because any mini-robot that is placed on it will be able to see camouflaged enemies. Naturally, Missliler Bots are commonly placed onto them.
The Atomic Bot's explosion covers the entire screen, dealing massive damage to all enemies on screen and completely depleting it of spikes in the Iron Fortress levels. An upgrade makes it powerful enough to kill a Giant in 1 shot.
Both place on upgrades for the Gunner Bot and Cannon Bot. The Gunner Bot gets a powerful gatling gun, while the Cannon Bot turns into a powerful Mortar Bot, which can fire Mini Bombers at any enemies infront of it. Of course, you only get these as the very last two upgrades in the game, right before the final levels.
Bubble Tanks Tower Defense has the Mega-Tower, which is four towers rolled into one highly powerful weapon that functions depending on the towers that morphed into it. The MegaMega-Tower is four Mega-Towers rolled into one.
The second game had the "improved sell-back" and "cheaper ghost tower" upgrades, which allowed players to actually make a profit out of building and selling tier 1 ghost towers. Admittedly it takes ten iterations to afford a single basic tower, but since the game won't start until you let it, with a lot of patience and mashing the 2 and S hotkeys, you can garner a substantial early advantage.
Cursed Treasure has the Chilling Crypt, which has a chance to scare the opposing "heroes" out of their wits, causing them to run away. The third charge experience upgrade gives it a third charge, massively increasing its chance of scaring the enemies. They also can be upgraded to have a chance of freezing an enemy on the spot.
Cursed Treasure 2 has the Shattering Crypt, which act like Chilling Crypts in the first but start out with three charges. However, it can be upgraded to deal damage over time to frightened "heroes", AND they also get an upgrade that gives them a small chance on instakilling "heroes" they hit (including Champions, but not bosses).
There's also the Lightning Crypt which fires at a much faster rate, each hit also having the instant-kill chance.
It's quite telling that the toughest level in the game only gives you two spots to place Crypts.
The Gemcraft series has a concept known as supergems, which are specially constructed gems to maximize damage output for as cheap as possible. Such a construction is practically the only way to hold back enemies for all waves in endurance missions (Gemcraft 0 or Labyrinth). The exact construction differs among the different Gemcraft versions, but the most common method is combining a gem with another gem that's two grades lower.
In Labyrinth, there are two common tactics. First is using the Red Bloodbound gem, which increases damage based on a percent of total kills. Next is the Lime/Yellow combination, which hits multiple targets with a damage multiplier.
mini Tower Defence gives an expert rating on score. It also provides an egregious example, as the easiest method to get expert is to hold down the "send now" key to bring all waves in at once, and use big bombs (leaving mop-up to just a single tower.)
Onslaught 2 allows you to setup towers to create combination attacks. Earlier versions had a bug which allowed six nukes to be fired on the same frame, known as a nuke flower. While that is fixed, it was since replaced by careful construction of laser chains.
Puzzle Defense has the "Eye of Ra" tower, which costs 200 gold to buy from Al Jezad's shop and 50 yellow runes to place onto the field. It only attacks ground towers, but it also has infinite range and damage, practically guaranteeing an easy game until the air units come. Unfortunately, it fires at an extremely slow rate.
Puzzle Defense is pretty unique for a tower defense game. It works like any regular tower defense game, but players only get currency by matching at least 3 runes of the same color in a row similarly to Bejeweled. This allows skilled players to get huge combos and gain vast amounts of game's currencies which they can use to place their towers down.