Well, I beat every Mega Man game from 1 through 9. :P

I beat Viewtiful Joe on Ultra V-Rated and unlocked Captain Blue. That was awesome.

I got all of the medals in Sonic Adventure 1. Some of those were crazy.

RPGs aren't that tough usually (since most things can be solved by level grinding) but I beat Tales of Symphonia, including all optional bosses and bosses you're allowed to fail at, using half EXP.

Uhm, I caught all 150 original Pokemon. xP

I got the highest possible rank in Elite Beat Agents. That involved beating every song on Hard Rock mode without missing a single beat.

I got all of the stars in Super Mario Galaxy as both Mario and Luigi. That was a bitch.

There are probably more, but that's all I can think of at the moment. xP