For me, Doom 64. I like that it's a new game rather than just another port, it lives up to the pedigree of its legendary predecessors, and unlike most 3D games of that generation, performance is rock solid. (Which I especially appreciated as someone who played Doom 1 on the SNES at 15fps, lulz)
Honourable mentions go to Turok 1 and 2 for their awesome weaponry and feeling so ahead of their time. You can see in both the seeds of Metroid Prime which a lot of the same people would go on to make.








