where ever you chose I'd also stay in Bangkok for a few days. If you want a summer destination I'd stick with Koh Samui( but avoid Chaweng). If you can go from Nov till about easter you might like to try Krabi. Are you really just wanting one destination or happy to try two or three?

I was very impressed with Ko Lanta. Head south and it gets much quieter. I stayed in Khlong jaak ( jark ) Its a nice little village with a few bars, and its got 2 huge beaches with barely anybody on them when i went at the end of Nov.
Or
Last year i went to Ko tao, which is pretty busy, but has a really nice vibe. Its not quite as rural as lanta, but its not as touristy as, say, ko samui.

Koh Samui is lovely. We stayed at one end of Chaweng beach and loved it. Plenty of bars and restaurants 5 minutes walk away but during the day just peaceful, yes, lots of people on the beach but that's not surprising.
I didn't like Bangkok, way too 'busy' and polluted. Some cheap shopping but overall I won't stay there again. Did Pattaya last year, hotel was one of the best we've ever stayed in (Sheraton) but Pattaya itself isn't great. Am going to Hua Hin in a couple of weeks time so looking forward to that.
If you want relative quiet I would suggest one of the smaller islands and steer clear of the towns in the main resorts ie: Phuket, Krabi and Pattaya, although there are obviously quiet places all around if you want that.

TC

I would suggest the Krabi area, there are still some relatively quiet areas left - its less developed than Samui, its one of the most scenic parts of Thailand and has a number of land based attactions as well as the offshore islands that people take trips out for beaching on a daily basis. Even Phuket has its quiet resorts and it does have some of the best beaches in Thailand. Khao Lak is also worth a look.

Bangkok is a great place to visit, as is the north of Thailand. Northern Thailand away from Chaing-Mai is relatively untouristy with some lovely countryside and friendly folk.