Quake 1 at the moment. I forgot how bloody brilliant that game is. I hadn't played it in several years, and even after games such as Half-Life 2, FarCry, and Doom 3 it still looks good and plays well. The level design is phenomenal. I'll probably play through Quake 2 next- that was a favourite of mine for awhile. I hear Quake 4 was dreadful in contrast to the first two but we'll see. It sure looks nice but the demo seemed way too contrived for my liking and took it's self too seriously. I'm generally an RPG player, but Dungeon Siege and Diablo are just about the only good RPGs available for Mac and I'm hooked on Gothic 3 for PC anyhow.