Check out the store on this site and look around. You'll probably get a myriad of opinions from people. When you say holster, that usually means belt clip, so I'm a bit confused. I like the Vaja holster, but it doesn't seem to protect the iPhone very much.
I have a timbuk2 soft case with velcro fastener that goes on your belt or on a backpack/bag strap. This case was originally made for my iPod G4 but is a great fit for the iPhone.
The neat thing is that with the velcro opening the iPhone is very secure in the case.
Also, it has a bet loop so you can wear it on your belt. The belt loop is attached to two overlapping velcro straps that you can also use to secure the case on a backpack's shoulder straps, a messenger bag strap, or even place it horizontally on your belt.