There are a number of threads discussing this that may help you. Like quite a few people, I'm partial to Rainbow Reef Dive Center -
my trip report from Step. 2013. You can get in 2 2-tank boat trips (4 dives total) per day. Guides are put in the water at no extra charge. A 10 trip (20 dive) package deal when I went drove the price down to the cheapest boat diving I'd ever done.
Different strokes, different folks, though. Another op. that's been spoken highly of is Quiescence. If memory serves from what others had to say, smaller boats with fewer divers and you don't get the 'free' guide service.
And some people love Conch Republic. I've not dove with them, but an op. that's spoken of often and so fondly merits you take a look at it.
Here's a December thread on
Best Dive Operators in Key Largo.
From August 2014,
Advice for Newbies going to Fla Keys.
While a dive op. may offer a scheduled dive site plan, this is tentative and subject to change, such as due to conditions.
I found Snapper Ledge really fishy. The actual winch on the bottom at Winch Hole was neat to see and take a picture of. The wreck(age, really) of the
Benwood was nice.
If you want to dive the deep wrecks such as the
Spiegel Grove and
Duane, have an AOW card, not just an OW card.
Richard.