SSBM is probably one of the most replayable games I've EVER bought/played. Theres always room to improve on your style, no matter how good you get. Shneaky, I've seen you in Counter Strike xD. You were kicking serious butt last I saw =P
Prolly CSS, some may beg to differ. But the best value game would have to be GTA: San Andreas. That game provided so many hours of entertainment.
Hmm i think guild wars, since i spent the longest time playing that and i am still playing it. Over 15 months play time i think.