I would have said F3, but the PC version was so buggy and unstable I took the hit on graphics and bought it for the 360. So much more enjoyable being able to play without random c2ds, stuttering, hanging, sound drop outs etc.. Only thing is I fought against them for so long I practically completed the game before it drove me to insanity.
Oh well, at least I get to play through it all again as an evil reaver on the 'box.
At the time it would be Asherons Call. Now... hmm... well... that's a hard one. The most time I've spent on a game in the last two years is with Fallout 3 - so that's my favourite 'modern' game I'd say.
M.