If I had to pick one, definitely Rayman O. When rayman came out for N64, I rented it like 24 hours later from blockbuster (oh ho ho), and I remember getting lost out of my mind at the first level.
Also, what the heck was with not being able to skip that one mile cutscene in the beginning every time? Was I just stupid when I was little, or did they really do that with the 64 edition?
Anyways, now that I'm older and can figure these things out (the internet is also a nice addition since then), I had an absolute blast playing the game. I've always been big on platformers (Ratchet & Clank still my favorite of all time), this one was amazing. Highly recommend it if you're into the genre