Having just got back from Thailand, I have to second the above nomination. Farkin awesome place. I only had a limited time there so mainly took in the beaches, plus managed to coincide with a full moon party - one of the best party experiences I have ever had, and I've had a few! Dancing knee-deep in the water to cranking beats with a bunch of newly-acquired best friends whilst taking in the full-moon - I'll say no more!
One thing I've found on my travels is that the enjoyment factor of a destination is very much dependent on the people you meet - I've had the good fortune to cross paths with an awesome motley crew of intrepid travellers that have greatly enhanced my experiences. Fate, luck - I don't know what you call it, but, it continues to astound me how a random chance meeting, that often would never happen if you were not in a certain exact place at a certain exact time, can often lead to a lifetime friendship.
If you do go to Koh Phanagn for a full moon, one suggestion I would make is to hire a jeep instead of the popular scooters - although the roads are all mostly paved now, you will be surprised by the number of people you will see sporting injuries from coming off a scooter. Plus, having a jeep is a great way to make new friends by offering them rides to and from the party.
Definitely check out Phi Phi if you can - well worth it. True, it is becoming more and more commercialised, but the reason for it's popularity is that the surrounding beaches (easily accessible by boat) are some of the most breath-taking in the world. It also has a great nightlife - check out Rolling Stoned for great live rock music, and conveniently located right next door, Tiger bar for house beats. Say wassup to Belle, the lil' Thai dude who works there - awesome dude, helped us get out of a certain predicament one night. Then later on, head to Carlitos for more house/electro on an open dancefloor overlooking the beach - but beware of the odd Swedish pop anthem they play - which is made up for by the number of Swedes there (see below).
In Phi Phi, and Thailand in general, you encounter tonnes of Swedes - make the effort to get to know them, they are a fkn cool race of people! Instant party - just add water. The guys' sense of humour is a little bit gay-orientated, which does take some time to get used to, but as long as you are not a complete homophobic you should be sweet. And as for the girls....Smokin'!
My only complaint - although admittedlt it is not strictly limited to Thailand - is drunken English blokes who want to fight you late at night in bars...WTF? Who the hell wants to get into a fight in paradise? I found the best way to deal with it was just to tell them to meet you at the Muay Thai gym the next day to go a few rounds - perhaps not surprisingly, of the 1/2 dozen or so I suggested this to, none of them showed! The one benefit was that it did inspire me to do a few hours of intense muay thai training everyday.
Thailand - as close to heaven on earth as I have ever been. I'm already planning my next trip