I can beat Donkey Kong Country on the SNES without losing more than maybe 5 lives, I've probably beaten the game 20 or so times and even speed through the levels as fast as I can sometimes. DKC is my favorite game of all time, and I pop it in every other month or so. I know all the secrets, all the extra live locations, all of the enemy placements, you could say I'm a bit obsessed with the game. I also proficient in Donkey Kong Country 3 in the sense that I can beat it fairly easily and know where all the banana birds, Kong Koins and Bonus Barrels are. I like this game, but I think I've played the GBA port way more. I'm proficient in DKC2, but I never really got into this one, I don't even think I've beaten this one 102% yet, it's my least favorite DKC game. Returns and Tropical Freeze I've beaten 100% but without the speed runs, and while I'm good at the games, I wouldn't say I could help you get through them.
LEt's see, I'm also very good at Super Mario World and I've mastered that one on both GBA and SNES but it seems like a lot of people have, since it's probably the best Mario game around.
More recently I've beaten Saturday Morning RPG, and while it is a pretty short game, there are a lot of strategies to utilize to survive battles. To beat the game completely you have to revisit chapters or "episodes" (as SAT AM RPG refers to them) to complete missions, gain achievements, and get S rankings. I'd consider myself well versed enough in the game (since I've played it so thoroughly recently) that I could guide anyone through it.
Awesome topic, it's fun reading what people have to say, and can't wait to see more!