Tailoring is generally takes quite a bit of time to really find yourself one that works and is cost effective. I've been through about 10 tailors before settling on the kind of fit I like, the cut, the details (felled seams so I can roll up my sleeves and still look neat, collar with collar pockets so I can use magnetic collar stays, fused placket so that the collar doesn't droop, darts at the back to extend in case I gain weight etc.).
I used to do most of my tailoring in Singapore and KL and I prefer buying my own material. In fact you should check out Seng Heng Textiles in People's Park - its the one with the food market on the first level (I actually am slightly skeptical about releasing this info for fear that retail prices of their cotton will go up). It's in level 2 and run by an old uncle and aunty. They are wholesalers and supply the tailors locally. Very wide range of shirting material and a small range of poly material for pants. The material for a 2ply 80yarn pure cotton goes only at SGD12/meter. However, I suggest you wash this before bringing it to your tailor as it would shrink by a bit. I used to tailor these shirts at Vivo Seah Im Tailor (at the back of the seah im Food Court). Again the uncle does an ok job, but his shirts are sent to Batam and they are stitched there. The fit is pretty ok and the seams are felled (double stitched), but you can tell that they are not done properly and start to crumple after a few washes. It also lacks a few minor details that I don't really bother about since it is only about $40 for the tailoring. It takes about a week to 10days to be ready for collection.
In KL I've used Lord's because my boss recommended it and we went together. The work was very good but a little too pricey for me. So stopped after 4 visits.
I buy my pant and suit material in Jakarta (for those keen) at Surya Jaya Makmur in Pasar Baru. They have very good wool blends at a fraction of the cost!! I get 75% wool blends at $18/meter. Now before you find it too good to be true, these are not entire reams, but rather left over pieces from old stock, the tail end. But still you can occasionally get really really good finds there. Once I got a 3.5m Versace 70% Wool for less than 50SGD.
Now to Bangkok, I've heard some very good stuff about how tailoring is very cheap there and have seen a few suits for friends made for best men and groom at their weddings. The quality I saw wasn't too good compared to the tailor in KL, for example, the shoulders weren't really perfectly done and the chest piece wasn't nearly flowy enough. But decided to try my luck in BKK anyway.
Found a good tailor called Charlie's Design as they were coming to Singapore on a travelling tailor segment. Called them up and fixed an appointment. Its run by two very nice guys called Joe and Roger. Was skeptical at first and just wanted one shirt done, just to see how it would turn out. But looking at their prices, I went and did 3. They even took SGD and converted it at the Super Rich (one of the best exchange rates in Thailand) exchange rate. They also accept credit card.
Their selection of material was really good. And Joe was very honest in telling me which was a cotton blend and which was pure cotton. The fir turned out to be really amazing. I got felled seams (done with a proper machine) where they don't wrinkle after a few washes. I got Split Yoke (if you follow Gentleman's Gazette, you'd know its the mark of a quality shirt) - their split yoke on a check print I did was aligned perfectly. It shows their tailors pay attention. They even gave me gussets at the side at no charge. The fit was perfect and the tailoring stitches was really fine even on the insides. You can see the workmanship that goes into it (ask for the VIP service). Their prices are very very competitive. It makes more sense getting the material with them for the shirts. It cost me 1600Baht (66sgd) for a pure cotton 2 ply and 1200baht (50 sgd) for a poly blend. In Singapore it would cost me 18(material at SengHeng) + 42(stitching Seah Im) = 60SGD for a 2 ply 80yarn cotton. But the extra $6 I pay in BKK is way worth it for the very fine workmanship. Plus they have a very good selection of material too. In comparison, a 2ply 80yarn shirt at a Singapore tailor would cost you about $120. (Now you see why I prefer buying my own material beforehand if tailoring in Singapore). The work was done in less than a day. 2 days at most. Even if you're in BKK for a short stopover like I usually am with work, you can pick up the clothes a week later in Singapore.
Their suit and pants, while they have a really good range, the material cost is a little higher for me because I can get such good deals in Jakarta. If Jakarta was not an option, it is a really great deal though. Again no complaints on the fit. I gave them my fabric and they tailor the pants fantastic. I may have gotten a really good pricing on the tailoring (its way cheaper than Singapore), but that may be because I have made a few stuff with him already. So for pants they range between 1500-3000baht (depending on fabric and wool content). Tailoring is way less than that, but again please check with them first if they can take your material, as they do for me. Many tailors wont be able to accept the finer wool (90%) materials you buy from Jakarta, as the handling is a little different. But at Charlie's I did not have an issue.
The good news is, usually you get it right the first time with Charlie's Design. In Singapore it took me a few tries with different tailors to find the one I liked best. If you only have one tailor you can go to, take the risk with these guys. You won't be disappointed.
I haven't tried the other tailors mentioned on this forum, so I cant comment on them, but I feel that since I already found one that I liked with everything I could get in a shirt at a good price, Im sticking to them.
I'll be doing a couple of suits with them soon, Will update again on the workmanship and fit. The last visit, Joe told me they were bringing in shoes as well. Possibly comparable to the Oxfordable and Straits Establishment shoes in Singapore. They looked really good. Am waiting for more info.
Location wise, its opposite a restaurant called Long Table. If you grab there. Walking is a short 5-6 min walk from Terminal 21.
Hope this helps.