Depending where you're staying on Ambergris Caye, I would say with "Amigos del Mar" or "Belize Academy of Diving".
Both of them have splendid boats, gear and have extremely good staff working for them. Just to let you know… The owner from Belize Academy of Diving does most of the dives himself, witch I think is very positive.
Both off them gives great services where I could put my hand into fire for.
Unfortunately I cannot say anything about the ProTech dive shop as I never been diving with them, so "?" about them…