Third on Phantasy Star 4; best game on the Genesis to me. At least, the one that took up a good majority of my time as a child.
*looks at an old bookshelf full of genesis game boxes*
Phantasy Star 3 wasn't too bad either. I got both of them at approximately the same time so I much preferred 4 over 3, but I went back and loved finding some of the ties between the two, great storyline overall.
I was mostly a Genesis kid growing up; here's my little list: Truxton was a pretty good vertical-scrolling shooter. Marble Madness was... well, maddening at some points, because you had limited lives and no continue/save point system (interesting game nonetheless). James Pond 1 and 2, while being two different games, are both fairly good, with lots of humor injected; nowhere near as much as Bubsy Bobcat, Sega's own internal competitor to Sonic. Terminator 2 (T2 on the box) was a good version of the "House of the Dead"-esque arcade shooter games, where you control the cursor and blow things up. Wonder Boy in Monster Land, Coolspot, Toejam and Earl, and Kid Chameleon are all classics that (in my eye) have stood the test of time, considering how long ago they were made. Earthworm Jim is still the best game ever. EVER.
It's fairly old now, but the game Panzer Dragoon Saga for the Sega Saturn was (and still, to an extent, is) my most favorite RPG of all time.
Heir of Zendor is a fairly obscure title for the Saturn that I remember as being a pretty good turn based airship combat game, for as little as I liked turn based games at the time.
Goldeneye/Perfect Dark for N64 set the bar for FPS's today; nothing like a return to the classics (and seeing that FPS's really haven't progressed past a system with 4MB of RAM)