My favourite game-playing experiences are in games without missions and storylines - highly replayability such as a soccer or driving game or an online RTS. One of my favourites was playing the skirmish mode on Warcraft II, but it always felt a bit limited and these days I think it would seem dated.
So I'm after a classic RTS - the same sort of style as the classic C&C/WC/TA games though it doesn't have to be old - which offers great skirmish (offline multiplayer) functionality. An active online community is nice but I hate APM games and find that really only playing against AI lets me indulge my preferred playing style without getting frustrated. So the AI needs to not be too repetitive and dull.
Obvious candidates seem to include Warcraft III or Red Alert II but it needs to run smoothly on Windows 7 and not look like crap. But if there are some indie games out there - even ones you've been involved in yourself - do share them. Perhaps there is a perfect game out there for my taste which I'd never heard of...