It depends on the the focus of your trip.
Pattaya
Like Evilectus said, Pattaya would be the place to go if all you want is to have fun nights with girls. But sadly, there is little to do in the day other then a little sight seeing, and probably more fun with girls.
Phuket
I believe Phuket is still better suited for a family vacation. With the many resorts around, beautiful scenery and the like, you will be able to find girls and clubs there no doubt. Everything there cost a lot more though, so unless the recession did not hit you, or you just have big pockets, then I wouldn't stay there for long.
Chiang Mai
I guess you will have to fall on Evilectus and other bro's word on Chiang Mai, as the last time I was there, was when I was a kid. I do remember lots of sights to see, and the guide asking my dad if he wanted to see a "tiger show", which as a kid, I wanted to see as well. Of course, what he really meant to say was...
Bangkok
I've been to BKK the most, and still, there are areas I have not explored and checked out. In terms of things to do, BKK is a city, which means you have what is available in one, like Singapore. Getting around is cheap with taxis (Just ask to be charged via the meter if they don't already do it), and cheaper with their MRT system which is exactly like Singapore's. So, shopping, clubbing, getting girls, it's all very easy to do. Lastly, since it is the capital, it also has became a hub to the other areas like Chiang Mai, Koh Samui, and so on, so you can hop onto a bus, or plane, to check out the other areas.
Hope that helps some
