For Thai massage, Healthland is a fine choice. You can call ahead to book, and the reception desk speaks enough English to take appointments.

Just keep in mind that Healthland does 2-hour Thai only - no 1-hour option. They also do the massages in separate rooms, which I like since I feel free to play my own music (not loudly) via speaker the way I wouldn't if I were in a common room separated by curtains.

I like the one in Asoke, since it's easily reached from both BTS and MRT, and it doesn't seem to get as busy/crowded on weekdays.