The implication that when Twilight tries to summon "The Best Defense Attorney" the spell Literal Genies Phoenix is pretty impressive, as it implies that Phoenix is the best Lawyer in the entire multiverse. Nick himself even points out the idea in Part 1. The actual spell may be a little more ambiguous but if Celestia thought he was good enough...
Rainbow Dash likes the idea of it.
Rainbow Dash: Awesome! It's only fitting, The best Pegasus ever gets the best lawyer ever! You were destined to defend me!
Every time Nick schools Trixie in court in Part 2.
Every time Trixie, an untrained prosecutor, manages to get the better of Phoenix, a trained Defense Attorney with an impressive track record.
There's also the fact that she tries to keep her cool every time a contradiction is pointed out, always coming up with some excuse or compromise to them.
Apple Bloom hiding a key piece of her testimony from Trixie, because "I don't like you".
Ironically, Phoenix managing to convince the court that Fluttershy,of all people, could also be the suspect with what little he had to work with in both evidence and witness testimonies was a pretty awesome move… Yes, a Shoot the Dog moment is also an awesome moment.
"OBJECTION! This can't go on!" "The... defense is objecting to… ITSELF!?"
Followed closely by one of only two times Phoenix gets chewed out. The second being when Applejack said that she should hit him in the head.
Not to mention that Twilight was considering firing Phoenix for his stunt. Yes. This just shows how deeply she feels about her friends and how it'd be more trouble for her if the lawyer who was said to help get Dashie out of a pickle is putting more of her friends in said pickle.
Phoenix telling Sonata why he's still defending somepony he has no stake in. "Did I stutter?".
Derpy's poem that calls Trixie a "Big, stuck-up… meanie."
Apple Bloom sneaking out the letter half from Fluttershy's cottage while Trixie was focusing on Twilight. Bonus points for the premises being under investigation with the potential of an officer busting her.
And later, she helps Twilight acquire two other pieces of evidence; a bag in the Everfree Forest, and a strange rod from a lake in Ponyville (which turns out to be Pinkie's golf club, completely fried). Both of these eventually came in handy and wouldn't have even been known about if not for Apple Bloom.
No matter how you look at it, murdering some guy who decides that in order to win, he has to blackmail using embarrassing photos or threatening to pull someone's sister out of life support is probably worth the trip to the sun, even if they have to cheat their way out of getting punished... Although considering the circumstances, Karma is the one who is awesome in kicking Ace's ass.
Gilda giving a very heartwarming speech about her friendship with Rainbow Dash after Phoenix sees through her faulty testimony... Then ending it with telling Phoenix exactly what she thought of him and flipping him off as well. That's Gilda for you.
In Part 4, who pulls the obligatory Big Damn Heroes moment just when everything seems lost? Surprise, it's Fluttershy again!
And it ends with Phoenix being hot upon Sonata's trail, especially now that she's to be called to the stand. The way he speaks of his oncoming confrontation with her, it reeks of awesome, considering the shit she put him through.
"Even if she tries to have me arrested for my intrusion, I got her on the stand. It doesn't matter anymore. It's time she pays her dues for all she's done."
And as it turns out Trixie is the one who manages to expose the blackmailing, solving it through some investigation that even Phoenix admits that he should have thought of.
Trixie declaring that Gilda's dug her own grave with the lies in her testimony, and refusing to help her.
Phoenix ruled in part 4 making the most out of Gilda's short testimony that Trixie prepared that gave him little to work with in questioning to turn things about to the point of Trixie and Gilda turning on each other.
2x multiplier. The leitmotif returns (not heard since Trixie's hammy debut in Act 2/4).
3x multiplier. Trixie doesn't stoop to cheap insults to snap back at Sonata, contenting herself to reveal the information Phoenix needs to continue the trial.
4x multiplier. Twilight catches on to what Trixie is talking about... and she lets her finish, right on the heels of Trixie's Caps Lock insult at her bangs. She just... brushes it off.
Phoenix gradually figuring out that Sonata hit Ace with the golf club is awesome for Phoenix repeatedly shutting down objections, especially from Trixie. He even literally schools Trixie on Ponies 101. And then he gets to throw her own words right back at her:
Trixie: B-but this is wrong! She couldn't have done it! Why would she? She was his assistant!
Phoenix: Weren't you the one who said the motive doesn't matter, Trixie?
Phoenix: But unlike you, I think it does matter, and I'll gladly explain what I think her motive for striking him with the golf club was.
Who would have thought that this video would actually make use of "right to remain silent" when that's not often used in shows like this? Very nicely included.
Phoenix stopping the trial just before Rainbow got a Not Guilty verdict for the sake of defending Sonata. It really shows how principled of a man Phoenix is when he's willing to make sure the antagonistisn't punished for a crime they didn't commit.
Best. Attorney. In. The. Universe.
Better yet? This mirrors a scene in canon where Phoenix does the exact same thing. In Turnabout Reclaimed, Dual Destinies' DLC case, Phoenix proves the supposed killer's innocence, pointing out that instead of pushing the victim to his doom, they had grabbed his hand in order to try and save him.
For the coup de grâce, Sonata sees Mia's spirit appear right next to Phoenix, objecting alongside him much like she would later do in Trials and Tribulations. But unlike that instance, this time they are proving her innocence with a bang.
You remember that summoning spell for "The Greatest Defense Attorney"? Well in The Stinger we find out that Celestia tried casting it to find help for her student and wound up summoning the ghost of Mia Fey. This implies that Mia is the real greatest attorney in the universe — but seeing as she taught Phoenix everything he knows, it's no surprise she recommended him to Celestia. And her trust in him was more than justified by his performance in court.