Warzone 2100 is an RTS from 1999 that went open source at some point and can be played on just about anything. It's unit design functionality is really cool and I'm a sucker for post-apocalyptic worlds.
DOTA, or any MOBA. I'm an old-school RTS fan and for whatever reason these games slide off me like water off a duck's back, despite being told multiple times from different folks that I'd probably like them.