I visited Bangkok, Chiang Mai, Phuket and Koh Lanta, Koh Phi Phi, Koh Samui, Koh Phangan, Koh Tao.
Bangkok was my least favorite. Traffic is terrible and it got kinda depressing to be offered sex as often as happens to a white man when he goes there.
Chiang Mai is amazing. It's beautiful, has a huge expat community, the people are much friendlier than Bangkok and the food is amazing. It's really easy for foreigners to live/work from there.
Phuket wasn't my favorite, but I only stayed in the town, which doesn't have much of a reputation.
Koh Lanta was pretty, but the beaches weren't particularly picturesque. I spent some time working at a coworking facility (Kohub) which I can't recommend highly enough. It's a little dead in Green Season, but not too bad.
Koh Phi Phi is a lot more picturesque, but it's a party island and I didn't get a decent night's sleep in my time there. It's kinda like Vegas in that it's great for about 36 hours and then starts to get old pretty quickly.
Koh Samui and Koh Phangan were really pretty but also really touristy. I'm not sure I would opt for an extended stay in either. Koh Tao is a diving island with mediocre diving, so there's not much point going there if you're not learning to dive in some capacity. But it's a great place to get an Open Water or Rescue Diver certification.
If I were working a remote job while in Thailand, I'd probably choose either Chiang Mai or Koh Lanta to start. But it's easy enough to move around, so unless you've got kids, there's no reason to settle on just one place.