My favourite evil team is Team Galactic. First of all, it's because Platinum was my first ever Pokémon Game, and everything about it stays in my heart. Second of all, their goal was the best of all evil teams, hands down. Cyrus was freakin crazy. And it was a challenge as well. I also love the admins (the ones who had the planet's names, not the scientist ....). I could write so much about Team Galactic, but I'm so lazy xD
Team Skull, which was revealed very recently, is looking very promising as well!
-WORK IN PROGRESS-
Btw, the artwork on my avatar is from an artist called OMOCAT, you should check her out ;)