This should come as no surprise to anyone whose seen my posts in here: Mass Effect 2.
It did almost everything right for me. Science fiction third person shooter, character driven plot, fantastic writing, excellent settings, highly polished(limited bugs), variety of different gameplay mechanics(the loyalty missions), high replay value, etc, etc, etc. Only real flaw that stood out was the "main" plot, which was fairly weak. I mean, in its essence its about alien abductions, not exactly ground breaking material there. Regardless, every few months i get a itch to replay the entire series and the second one is always the one that sucks me in.
Max Payne 2 is another game i absolutely adore as well and have to replay once a year or so.