[Iceman]

This is both easy and hard. I'll keep mine to teams that I think would actually work together.

Avengers
1. Wolverine (leader) - He does what needs to be done. Would have gone with Cyclops but after AvX I don't think any Avenger would work with him.
2. Iron Man - At his best when he doesn't have to worry about controlling everything.
3. Mr. Fantastic - Great for the think tank and his powers are a great non-lethal option.
4. Beast - Between him, Tony, and Reed any problem is solved fast.
5. Iceman - "godlike" with his powers fully realized
6. The Thing - You gotta have a strong man and someone to keep Reed grounded

If you wanted a team that was nearly all brains you could switch out Giant-Man and Hulk/Banner for Wolverine and Thing.

Masters of Evil
1. Norman Osborn - Because he pure evil and crazy, unlike a lot of villains that think they are doing what is right.
2. Bullseye
3. Juggernaut
4. Doc Ock
5. Sabertooth
6. Red Hulk

I left Doom and Magneto off because I don't think they would work well with each other or Osborne. I think the nature of most super villains is to be in control, that's why so many have henchmen and run organizations/countries/gangs.

If was just the scariest and hardest to beat team of villains I would say

1. Apocalypse
2. Doom
3. Magneto
4. Venom
5. Ares
6. Dark Beast or Mr. Sinister