Don't know if this has been mentioned yet (don't want to read the whole thread)
St. Charles:
Mike's Guns - always very helpful, but prices seem pretty high. EX: 1953 TTC for $260, Cabela's $200.
Nassau: Pretty good selection if you like semi-auto's. Prices arent terrible, helpful people.
Gateway Outfitters: Haven't been in there in a while, took awhile to get someone to help me.
Bass Pro: We all know about them.
Cabela's: Not the best prices, but they do have a decent selection. I've bought 3 guns from them so far.
St. Louis City:
Bullseye: I will not go there again. Crappy range, only a couple gun rentals (4 1911's 2 Glocks), expensive range rental. Never again.
St. Louis county:
On Target: Somewhat helpful, EXHORBITANT pricing. TTC (see above) $350!
Denny Dennis: Huge stocks of ammo, not much of a pistol selection, sometimes hard to get help. I stood there for a solid 10 minutes looking at ammo (behind the counter), no one asked me if I needed help, no other customers around, 3 guys behind the counter. I just turned around and left.
Mid-America Arms: I love these guys, mostly because they're right down the street. Not a huge selection, but it changes often. Usually have ammo, but the shortage hit them especially hard. Lots of milsurp rifles. Very helpful people, decent prices. Last time I was in there they had 2 VZ scorpions and a Mac-10. Lots of shotguns, AR's, AK's, Mosin-Nagants for $110.
Outside St. Louis county:
Top Gun: Fantastic place, indoor shooting range, gun rentals (full auto MP5's, big a$$ revolvers, etc) decent selection of new and used guns, lots of accessories, usually always have ammo, friendly people. The range is clean, nice equipment.
Dunn's: Mostly a hunting/fishing store, but a decent selection of handguns and ammo.
Smitty's: Down hwy 67. This is almost the Holy Grail. John (owner) will stand and talk to you until you are absolutely finished. He helped me a great deal when I bought my P-01. Then he moves to the next person. May take a bit of time to get some help, but when you do, you get all you want. His parent's help him run the place, so it's just the 3 of them. Huge selection of handguns, rifles, shotguns, ammo, and accessories. Just a great place to deal with.