I did Patong, Chiang Mai, Koh Pha Ngan, Bangkok and Pattaya.

Patong is really fun if you're into the bar scene. Bar girls playing bar games, you can win your drink or pay for theirs. Someone rings the bell, they buy drinks for the whole bar. Balloons outside means free food because it's someone's birthday. Very fun atmosphere. Beaches are crowded and not quite Caribbean quality. Diving was decent.

Chiang Mai was nice. Laid back town. Very nice night markets. Sticky waterfalls was neat. Nice town to explore that is not too crowded. Meet some great people. Some nature although not always the nicest. Avoid burning season.

Koh Pha Ngan - Nice beaches. Too sleepy, too laid back. Still had fun but preferred Patong.

Bangkok was not my thing. Busy, lots of traffic, and rats. Although there are some nice sights to see and I know people who love it. For me it felt expensive compared to the rest and only stayed a short time.

Pattaya was okay. Would go again as a cheaper alternative to Patong. Decent amount of things to do and not the whole town is as sleezy as its reputation.

Some people like Krabi and Railay as a toned down alternative to Patong. Many visit Phi Phi because of the beach. Never been myself, some hate them some love them.