personally, and a few people will prob hate me for saying this, but, i found halo1 far more impresive than halo2.
Me and my friends in our school days spent many a hous playing halo1, and i mean hours!! we could sit and play through every map 3 or 4 times. My friends used to love playing halo against me [ i was a GC man at the time ] and all of them owning an xbox they pretty much wiped the floor with me, untill i got good
[ this is going somewhere btw ] We had pretty much played halo to death. and once halo2 came out we thought WOOOHOOO carnage time again, and for the first month or so we played it to death, i still play it now on Live, but i think because we played halo1 SOOOOOOO much, halo2 kinda got old quickly, and we'd get bored after 1 run through the maps, we tried editing game types to make it more interesting but it didnt quite have the oumph halo1 had. as for other FPS games, the only others i play tend to be PC ones, inc halo PC :P
but console wise, id choose as my favs:
HALO1/2
CoD2
Quake4
Doom3
and a few others :P