its hard to finger a single killer app on PC. If you aren't purely interested in top of the line graphics, then you have 20 years of complete gems just sitting there waiting to get played in almost every genre, adventure, action, FPS, RPG (both action and D&D style story focused based). If you're purely a console gamer since day dot, then you have an entire genre there left discovered (RTS).
I game on PC,ps3,ps2,wii,ds I have my favs for each console (generally geared by control systems I prefer for X game) but I do always return to some of the classics on PC when I want refilling. Currently playing through Baldur's Gate 2 again, graphics are cruddy but by-joy is the game itself steller (and its a reasonably new game, released 2000, built on a game engine designed ~1998).
