The three I currently remember having the most fun with:
1) 360 (multiplayer Gears1 through MW2 has all been great).
2) Sega Genesis/Sega CD - Lunar, Dark Wizard, to name a few stand-outs.
3) Sega Dreamcast - Resident Evil: code veronica, RC Tyco Racer, Grandia good stuff.
I didn't hate any of my systems, but the following 3 I owned the least amount of games for:
1) PS2 - When the PS2 was in its prime I was in college so while I enjoyed the games I had for it, fraternity and chasing skirts left little time for gaming.
2) Gamecube - loved Resident Evil 4 and Windwaker, but see above, had little time to buy much else.
3) XBOX - uh see above...loved Halo, Panzer Dragoon and even lesser known games like sudeki, but didn't really build a library.