Hey Eden,
I've tried a couple of tailors and researched a bunch of them. It depends on what you need but I can recommend Rajawongse on Sukhumvit just North of Soi 4/ Nana. 100% cotton dress shirts ~US$40 / 1200 Bht. Suits in pure summer weight wool or wool/cashmere with two pairs of trousers ~US$300 / 9000 Bht???? (bought them a while ago).
I also buy dress shirts and linen casual shirts from Tajmahal, who, by now have probably moved from their location further down the soi on Nana from the entrance to Nana Ent Plaza (NEP) to Sukhumvit, south of the gas station on the SE corner of Sukhumvit & Nana, near the pharmacy. Again price for 100% cotton shirts or 100% linen ~US$40 / 1200 Bht.
Raja has also had good reviews and is directly to the right of the NEP entrance but I haven't tried them.
Make sure you give yourself plenty of time for 2 or 3 fittings at the least for suits. Shirts are easier, usually initially measurements and then a try on upon completion.
Hope that helps.
Best,
SBDOTKU
