I've been out with (including only those ops still in business and all in Key Largo unless otherwise noted) Conch Republic Divers (Tavernier MM91), Florida Keys Dive Center (Tavernier MM91), HMS Minnow, Horizon, Ocean Divers, Ocean Quest (Islamorada MM85), Quiescence, Rainbow Reef, Scuba Do and Silent World (shop and boats located miles apart, 28'-30' boats/12 divers). All were at least adequate to varying degrees according to my subjective and varying requirements (i.e. owner/employee amicability, pleasantness, knowledge, organization, willingness to spend a few minutes with you to find out what you're looking for from them, etc.) Here's my druthers:
Six-pack: HMS Minnow-gets you to the good spots quickly and before the other boats. Capt. Jeff has a great sense of humor and will go the extra miles, literally, to put you where you want to go or where he thinks you should go, if you ask for his advice. Quiescence also is an outstanding six-pack op. I've never read any negative posts about either of these ops. For "medium size" boats (~28'-34' range) try Scuba Do (34', 10 divers max, lots of room). Or Conch Republic's "little" boat (~30'), AquaticDiver. Conch Republic is frequently mentioned here on SB, always with praise. They will gladly make sure you get any available discounts, e.g. "ScubaBoard Member," without your having to specifically ask for them. Bigger boats (42'-46' range) can also be had at Conch Republic (see previous remarks) as well as Ocean Divers. Ocean Divers will go out with two divers minimum although they can get claustrophobic on those Sunday mornings when a south Miami dive shop/club is present. I've only been out with Ocean Quest once. It was a good experience. Someday I'll get out with them again. As for all the other ops I listed; I'll only use them if these preferred ops I've commented on are unavailable.