Enclave was a great game, and not just xbox btw--it was released for the PC too. The same people who made it went on to make Chronicles of Riddick: Escape from Butcher Bay, and they have a new game coming out next year for xbox360/ps3 called Darkness that sounds really interesting, and at the same time really disappointing because it's not coming out on the PC too.
As for indie games,
Darwinia and
Uplink top my list. I think Darwinia is actually the only RTS I've ever bothered to finish. Uplink has been discussed on the boards before too, and I think the concensus is that it's best played with mods. I've tried SimplyUplink and Onlink; the former is mostly just a tedium-reducer that doesn't actually change the gameplay a whole lot, and the latter is a total conversion that is still in beta. Mods are available at
http://www.modlink.net