Summaries available 'cause I'm too tired to post something complex.
1) Lylat Wars (Star Fox 64)
I love this game. The real start of a series I now follow very closely, great idea, fun levels.
My only gripe is that this spawned the DO A BARREL ROLL thing.
2) Sonic 3 & Knuckles
Greatest two dimensional adventure ever. Very long game to complete to the end, three mostly different playable characters, lots of unlockables, etc.
3) Pokémon Ruby/Sapphire/Emerald
I loved this game so much. My playtime is in quad-figures and I completed the 'dex only recently. Hoenn is arguably the best region in the series and the trainer designs were better than most.
4) The Sims
The start of a revolution in simulation games, Sims is so expansive even without the packs and it remains one of the series' I follow closely.
5) Metal Gear Solid 3 : Snake Eater
The best MGS, it had much more features than it's predecessors, a very long gameplay time, and didn't feel constrained to the movies which is what MGS4 suffered from.