Do you want clubs or nationals? or both?
because theres a lot of obvious ones:
Barca, Real, AC, Man Utd etc...
Brazil, Argentina, England, France etc...
but honestly some teams like Porto and co. are excellent in the game aswell, every year me and my bro make our own Champions League and pick 16 teams each, the games between teams like Marseille v Barca are usually cracking whereas a Man Utd v Real Madrid end up being boring!
PS. I'd say Inter + Barca are the 'cheat' teams this year... Adriano can shoot from mostly anywhere and the keepers will let it on or fumble it, and Barca have the pace/power of Henry + Eto'o up front.