It's a very close tie between Monster Hunter Freedom Unite, TES3: Morrowind, Banjo-Kazooie, Dark Age of Camelot, Baldur's Gate and Dragon Warrior 2. Honorable mention to STALKER (with the LURK mod it would be close to number one but I feel as though that would be cheating). I would be hardpressed to pick just one but based on hours played the title has to go to Morrowind at roughly 5,000 hours over the past decade with MHFU and DAoC in second and third at around 3,000 hours each.
I'm sure if I was a tad bit older the list would contain more classic titles but it wasn't until about 10 years ago that I got really heavily into gaming. Until that point it was just a hobby with most of my really great gaming memories coming from playing with my father or watching him play. Then he inspired me to start collecting and playing classic games and the rest is history. Of course this list will change over time (just like everyone else's) but for now those are my favorites.
Playing - Borderlands 2, Hitman Absolution, Vanquish, MGS HD Col, Resonance of Fate, Monster Hunter FU, Demon's Souls, Shadows of the Damned PSN: DocDelicious