For Mexican, Taqueria style I think La Monita remains the best in class:

http://www.lamonita.com/La_Monita_Taqueria/Welcome.html

in Mahutun Plaza, ground floor, left, a short walk from Ploen Chit BTS (exit #2)


For pizza, the Pizza Aroy truck, evidently moves between two locations in Ramkhamhaeng, is supposed to be pretty good, but it is a bit out of the way and hard to get to.

http://www.pizza-aroy.com/