If you mean to ask what other FPS games I've played, note that I've been gaming religiously since the early 90's on PC...The notable FPS's in my gaming background include: TF, TF2, Doom, DN3D, Wolfenstein, CS, CS:S, Quake [for a bit], Half-Life, etc..But I've grown into the more competitive gaming scene on consoles (Gamebattles), and through Black Ops, I've met so many talented and high-ranked players who've become my closest friends...Hence my decision.
I don't think "best" is ever appropriate to use when discussing topics like these. Black Ops is my favorite modern FPS, it isn't perfect, but I'm not afraid to acknowledge it's flaws- that's what makes my statement an opinion. We all have different experiences and tastes, I'm sure you have your own favorites, as do I.
