Very hard to choose.
Original Tetris type A for gameboy, I sometimes just sing it in the subway.
http://www.youtube.com/watch?v=NmCCQxVBfyM&feature=related
Sonic Green Hill Stage on Genesis, my brother and I even composed lyrics for this heh.
http://www.youtube.com/watch?v=kR45O51N2OI
Lost Odyssey's Main theme, just superb, it makes me buy the game.
http://www.youtube.com/watch?v=q5vb6VFOiCI
And like many, Morrowind, Neverar Rising was on my head for months back at the time...
http://www.youtube.com/watch?v=DORtFo2MAE0&feature=PlayList&p=4BFF9D1A531D61ED&index=0
FF VII Crisis Core has a great soundtrack overall, I especially like "wandering on a sunny afternoon"
http://www.youtube.com/watch?v=Xpe72ZQY5Ec&feature=related
and Street Fighter 2 is legendary.
http://www.youtube.com/watch?v=cK1C0uj27xU
A shame that Street Fighter 4 had unimpressive music, at least the tunes from third strike had something unique... Dudley's "You Blow My Mind" probably being my favourite of these:
http://www.youtube.com/watch?v=Et4tOpJ63cQ&feature=PlayList&p=060C8ED7E5A95FC4&playnext=1&playnext_from=PL&index=22