Probably already mentioned.
I am very fond of "Love Scene" restaurant on Sukhumvit Soi 23. Thai food, Isaan-style. Best way to do it is go with two or three friends, order a variety of things, and do family-style. Great fun, VERY good food.
For Tex-Mex, Charley Brown's off Sukhumvit Soi 11, near Cheap Charlie's is very, very good. Be warned: their margaritas are POTENT! (If you do two, PACE YOURSELF! The second one will hit HARD if you don't.)