Here's my favorite curb. It has the swept back shanks, no joints in the mouth, swivel shanks so a quick 1 rein correction can be used if needed, and the shanks are very short (5 inches). In all, it's a very mild bit.
I have my mare in it for western and she goes back and forth, nearly daily, between this, a snaffle, and bitless. NP Sweet Iron Loose Cheek Western Curb Bit 5in - Horse.com
It does get rusty pretty quickly because of the materials it's made of so here are a few more "showy" bits with similar action and gentleness:
I think this one is just gorgeous. I really want one just so far I've been unable to justify it. The one thing with this one is that it does have the smallest amount of gag action. Your horse may or may not like that. The reviews seem to indicate that most horses like it, but who knows until you try it.
The shanks are also longer than the first one I posted, meaning that you'll be able to use even less rein contact to get the same result. SS Hinged Ported Roller Futurity Bit 5in - Horse.com
The copper roller will also encourage acceptance which is a plus!
Here's one very very similar to the first bit but a bit more showy. It has the really short shanks, no gag action, the ported mouth, all good stuff. I think this is the one that SMRobs (who's basically the forum's cowgirl captain) uses on most of the horses she trains as well as her own horses. AT Low Port Loose Cheek Low Port Western Bit 5in - Horse.com
Good luck choosing! :)