What are some of the best Pokemon you've ever used in a hack and why?

For me it would have to be Pokemon Wack's Charizardo. Fire/Meme type, with the ability Memetic. This means any contact with him can turn your ability into Memetic, which also reduces the stats of non meme-type Pokemon.

In a game full of opponents with insane abilities it's great to have this ability-negating friend in your back pocket.

Gyarados in any hack its avaiable with physical special split. Dragon dance + water/ice/ground coverage beats everything.

For some reason, a lot of hacks give out Riolus like candies and they end up carrying every game I've played after evolving.

I'm not sure if you meant "best" competitively - if so I'll fall short on that! I guess battle-wise I'll second what was said about Gyarados. When I read the title I at first thought about the most memorable Pokémon I've liked having on my team when I play hacks. My Lapras in Pokémon Orange was by far my best team player - it's a strong Pokémon in any game but I gave mine a Pinkan Berry and was so amused by having a pink Lapras I ended up hardly using the rest of my party lol. All these years later I've also still got a bit of a soft spot for Naranja's Crystal Onix even though it's not like you could get very far there as we all know.

in the drayano hack, blaze black nidoran m had a restructured moveset that gave it a lot of stall and status moves, I love my nidoking that's setup to put everyone to sleep, then there's spinda, much better than base game, a very solid choice now.

Snakewood's kenchukuo. I weirdly fell in love with the fakemon. It's absurdly strong with an insane 160 base ATK stat and a 165 speed + a freaking speed boost and of course dealing this monster is incredibly tough especially on that deadly seven part. The most satisfying part about this fakemon is that it has a belly drum which it could sweep every single pokemon in no problem.