Very tough decision. I'd have to say Parasite Eve found its way into my PSOne the most.
DB GT Final Bout
and so many more...
I'd have to agree with Sushi Pirate on the GT, and also add Road Rash. Gotta love it!
Resident Evil 2, followed by the other two entries in the RE series.
Silent Hill and Dino Crisis 1 round it out.
Sometimes, the worst PSOne game can oddly be the 'best' too.
I thought the M&M game was good, albeit with all the controls and goofy bugs.
What did M&M stand for? Sorry can't remember what game this stands for.
I'm thinking it just stands for M&M ("Mars and Murrie"), as in the candy.