Mario 64 and SSB 64...
I may have legitimately logged thousands of hours on those two. Back in the day I used to be able to 16-star run Mario 64 in the low-mid 20 minute range, which as I'm typing this sounds a lot more pathetic than it did in my head. There was also about a two year period where Smash was literally a daily occurrence. Wake up, go to class/work, come home, everybody comes to my place to play Smash all afternoon, then we go out at night, or just say screw it and play Smash until 4am. Wash, rinse, repeat.
Underrated gems tho;
Get Force Gemini
Blast Corps
Pilotwings 64
Wave Race 64
At the time, it seems like the narrative was that the N64 had a handful of fantastic games but an overall thin, weak library. I think in retrospect it was stronger than we realized!
And yeah, I think the 64 library aged better than the PS library overall, but we gotta remember that the PSX literally had like, 20 times as large a library. There's probably about as many still-golden gems on both, just a lot more garbage on the PS. Totally tend to be different gems too; alone in the dark immersive solo play, give me a PSX. Couple friends and beers, you better bring the 64. Other than the Wii, I guess, it's kind of a shame that the primary consoles since that era have all been so similar; I miss the days when the competitors had very significant strengths, weaknesses and more of a clear-cut identity. When I was a teen if somebody owned a PSX AND a 64, I was jealous. I can't imagine a reason anybody would want a PS4 AND an XB1.