My favorite is probably owc_Traverse, another community made map.
I dunno, I've yet to win with the Rebels on traverse, that barrel trap on top of the console objective always manages to wipe out half my team if we ever get that far (not very far at all).
Invalid Steam ID Ticket. Or something. Disconnected.
Apparently this is a common problem with source mods. I used to get it, but if you google the words "invalid steam ID ticket", you'll get about a billion other forum posts with the fixes, something about corrupted Steam files. Make sure you look for the fix that involves only deleting a few files, not your entire Steam install.
There was a big update just a few days ago, so if you join an empty server you can make it full in a few minutes just by sticking around, at least on the North American servers.
I won't say that this is the most balanced mod ever, because it really depends on the map and the people playing with you, but if the Rebels have a core of fairly competent team players and the Overwatch player is half decent then some pretty epic struggles are to be had.
This sounds an awful lot like Nuclear Dawn. Similar?
Actually, as far as I can tell Nuclear Dawn is a ripoff of
another Source mod called Empires. Except without vehicles. Nuclear Dawn looks like its about two teams of players, each one with a 'commander' player who directs the actions of the rest of the team and builds structures.
Overwatch doesn't have base building, so if you like that then you're out of luck :p. In Overwatch the RTS component comes in with the Overwatch commander, although I guess its less like RTS and more like Real Time Dungeon Master...ing. The map comes with a set of objectives that the Rebels have to complete, and the Overwatch player controls traps and enemy spawning, and gives orders to the spawned minions. Rounds generally only take between five and fifteen minutes.
It's also free. Just saying.
Also, Zombiez!
http://youtu.be/US_ob4dBMuc You've got to love a dev team who'll make a video like that.
ANOTHER EDIT:
Half-decent gameplay vid:
http://youtu.be/AaVnit7I2ncIts a testament to the dev team that that particular map was still fun even before it got a full texture pass :p.