Feel free to chime in with your own opinions, but here are some of the cities I've telecommuted from in thailand over the years and the pros/cons of each of them. Specifically regarding apartments.

Thailand in general

One of the best reasons to live and work in thailand, is it's the norm to get an apartment that is fully furnished. Whether you're a thai person or a foreigner, the apartments come ready to go. Only rich ppl are generally expected to have their own furniture. So unless you're spending over $1500/mo, you're gonna get some furniture at the rental price. Furniture is usually not bad in terms of generic ikea looking stuff.

This is refreshing compared to renting places in other countries where your only option is pretty much AirBNB if you want a furnished place, month to month.

Bangkok

Bangkok has good quality apartments around the $300/mo range. You will probably need to pull in your own wifi, because most shared apartment wifis I've lived at are a bit slow. If you just wanna use shared wifi, then see if you can bench the connection before you move in (might require buying a 1-day use coupon). You can pull in your own wifi from 3BB or True for about $25/mo. Also bkk has LTE which I don't believe exists yet anywhere else in thailand. While there is no flat rate unlimited pricing like in the US or philippines, the rates are also a lot more reasonable than both those countries when getting charged per-GB. Obviously LTE eats up the data quick though, so this might be best used as a treat when you feel like working outside your apartment some place special.

The upside of the apartments I've rented in bangkok are that they have great amenities nearby or even on-site. Just go down the elevator, and you could find a mini-mart, a laundry shop and/or machines, a thai restaraunt, a salon, etc. Many apartments (especially the thai-style ones) have room service running late into the night. The food doesn't even cost more than going down and eating at the restaurant itself.

The downside of apartments in bkk is the requirement of a 3 month deposit and a lengthy lease. The nicer places the landlord will want a 1 year contract. I've haggled nice places down to 6 months without too much trouble. The 2nd tier apartments would still want a 3 month contract probably. And while you can find month to month places, the monthly rate will be higher. Compare this to other cities around thailand that allow you to easily get in at a month to month rate without breaking the bank or asking for a deposit.

Chiang Mai

There are some killer deals on small houses or apartments in chiang mai. You definitely get a lot of bang for your buck. Maybe you could find a decent place for like $200/mo or an amazing house for $500/mo. Plenty of places don't need a deposit and are cool with you going month to month. Apartments and houses don't have as many amenities as Bangkok, but since the traffic is a lot less crazy than BKK, you could rent a moto and get around easily. Going down to 7-11 or the laundry shop isn't as big a deal when you have your own wheels.

Koh Samui

A bit pricey and closer to BKK's monthly rates. Also some places asked me for a deposit, but I could go month to month easily. $300/mo will still find you a decent place, although not right on the beach. The problem for me on Koh samui was the power would go out during heavy rain storms, which often occurred in the late evening which is near the time I started working. In any given month, the power and internet would cut out on me maybe 2 or 3 times. Kinda a hassle if you work a 9-5 job.

Pattaya

Similar nice apartments with great amenities just like bangkok, but at a cheaper price. Instead of paying $300, you could pay $200. Instead of paying $1000, you could pay $600. There's loads of apartments in pattaya, and most are pretty nice. They don't all require 3x deposits, and they're used to tourists renting them month to month. You can also pull in True/3BB if your apartment lacks shared wifi.

This may sound surprising, but I would probably rank pattaya as the most convenient city to telecommute from. In bkk, if you wanna take a quick break from working and have a drink with a friend.. you gotta jump in a taxi or skytrain and spend 30 mins and maybe $4 to go somewhere else in the city. In Pattaya, you can hop on the songtaews that go in an endless circle for 30cents, and be just about anywhere westerners like to go within 5 mins. Within my lunch hour, I can go out and get a massage, get a drink, or find some food. It's also a city that never sleeps so if you work night hours, then the city is ready for you on your schedule.

If you know anything about pattaya though, there are some major downsides. It's a pretty horrible beach to actually swim in (even though I enjoy having a coconut under the umbrellas after a long work day). There's sexpats and hookers everywhere (don't try to find a good girl in this city). And you gotta tell people you live in pattaya...