No matter how often I play it, no matter how long I play it for, I never feel like I'm bored.... Only one game I can think of that fits.
One Must Fall: 2097
But only in tournaments - training up a custom character and outfitting an ultimate 'bot of doom was just awesome. And you could 2-player against an opponent's custom and see who wins (usually the one with the most upgrades, but not always)
Although I have to admit, I can spend quite a while playing Prototype as well. Funny how no RPGs are on my list though, I love playing them, but I get bored quickly if I'm sitting in front of one I've already finished and just levelling.