Hi all...need help as to the most comfy (read air-con, clean beach, dive friendly) resorts to stay in all the 3 islands in the month of July. We are a family of 4 divers, also which island is worth a longer stay?? Confused as always with so many choices...

For divers, I'd say spend your time in Koh Tao. www.sawadee.com is a good place to start looking at accommodation in all 3 places.

Koh Tao certainly if you want to dive, you can go over to Koh Phangan or Samui too, KPG is laid back, Samui is very busy, lots of tourists in July aspecialy later in the month.
You can either fly to Samui from Bangkok or Singapore but you can also do an Express Bus/ferry combination with www.lomprayah.com from Bangkok for around TRHB 850/1,000 each if you are on a budget. Long journey but ok if you are happy with that, an idea is to go to Koh Tao on that combo ( if it suits your style ) and then perhaps fly back from Samui.
Coral Grand Resort is about the best unless you go for one of the new expesnive villa places, http://www.beachsiam.com/koh_tao.html has info, make sure you also visit Koh Nan Yuan, sensi on Koh Tao another good place, many have a choice of fan/air con but the higher up the price range you get all air-con, http://www.r24.org/beachsiam.com/kohtao/sensi/review/ has reviews, as a family might suit you a lot better than othes as they have Family Houses, do not that some accommodation has resticted electricity (!) so do watch for that on the descriptions, Coral Grand though pretty sure has elec 24hrs.
Depending how long you have adding a few nights on KPG and then finaly on Samui for some shopping and livelier time is quite easy, other boats link but Lomprayah is 'fast' and links all 3.

Thanx...much appreciated!!

I'm not a diver but I'll skip Koh Phangan. It's no charm and doesn't worth going unless you're really into full moon party, half moon party, black moon party. they way that their booking and business are centered around these parties should give you some hints on what this island is like.

for samui, i've stayed in muang kulangpan hotel in chaweng beach worth i think worth exploring as it's sort of a boutique hotel at a lower price range. relaxed and charming. and it's got coral reef at the beach.

for koh tao, i've stayed in charm churee resort/hotel. those rooms/huts with seaview might be better than beach-front ones (too damp in my view).