My two favourites are The Lost World (PS1) and Operation Genesis, but since I can't pick both, The Lost World gets my vote. I play a lot of platformers on consoles both old and new, and TLW is still high on my list for it's graphics (good by PS1 standards) and the option to play as both the dinosaurs and the humans. The only thing I didn't like about it was the inability to save your progress to a memory card, but I've still got the passwords for it so I can pick whichever character I want to play with each time I boot it up.
I still like to play Operation Genesis on PC every now and then too - back when it came out I was thrilled at the idea of being able to create my own Jurassic Park from the ground up, and taking control of the ranger copters and safari land cruisers whenever I wanted to. Started playing it again last week, and so far I've gotten the park's rating up to 4 stars. Managed to raise it up to 4 1/2 at one point, but the game crashed before I could get round to saving it