Hmm... This is a tough one...
F.E.A.R. wins for atmosphere, but UT2004 wins for multiplayer entertainment... and then there's Crysis for how many different ways there are to approach the levels... Deus Ex wins for story...
Ugh, I give up, I just can't choose one "best" game...

How DARE you brand Deus Ex as a shooter?!

CoD:MW2. Someone had to say it!

How's the game? Considering buying it once I get a shitload of cash but I'm hearing chatter about the game rife with hacking and shitty lag and people jumping into games and suddenly getting level 70.

Well, the hacking is kind of on a week to week basis. BUT...yea the sudden level 70 thing just happened to me and it sucks.
I have it on the 360 too though and it also has had its share exploits. I can't make any excuses for that and even if rare it is annoying.
I believe it is actually a side effect of the game being so popular, much like counter strike.