For me the nostalgia glasses are turned up to the max for Crash Bandicoot.
I love the character designs, and the music is an absolute bop to listen to. (Got the first games theme as my ringtone even at 27!)
And I enjoyed a good challenge with some of its levels. The bosses were neat and I thought it was just an unusual neat idea that you jump on boxes to progress rather than punching a block for a fireflower.
You?
Probably Mario 64, just because it was so revolutionary at the time.
It created a genre. And it was surprisingly competent at what it sent out to do. In hindsight it was a miracle. Brand new technology harnessed to birth the 3D platformer. It was amazingly ambitious all around.