I'd have to go with a Nighthawk Custom GRP in .45 ACP. I've never shot one, but I held one today. It was amazing. I needed a cigarette afterward.
However, an HK Mk23 with a threaded barrel would do just fine as well at $1k less. And for a cool grand, I'd be inclined to go with a threaded and railed Sig 1911.
But if I just had a few hundred bucks and high-capacity mags were a must, I'd settle for a Glock. I know a lot of people love them and say they're very reliable, but I just don't like them.