The Safariland 6000 family are very nice (6004, 6005, 6074, 6274, 6304, 6305). I got a used 6004 for a Glock 20/21 off ebay for $40 and it works great after you take the plastic sleeve off of the tensioning bolt.
The Blackhawk platform is nice. If I had money, I'd probably get a Blackhawk Serpa CQB with the drop leg setup. Fobus makes one that's pretty cheap, and buying stuff made in Israel makes me feel all warm and fuzzy inside.
Unless you take a lot of carbine classes, or compete in 3 gun, drop legs are pretty limited.
If you have a lot of money, get a Safariland or a Blackhawk. If you're poor like me, I'd get whatever you can find used and cheap. You probably won't use a drop leg as much as you think you might.