Pico's on Bellaire. The Big Ass Margarita is truly a big ass drink. They use hand-squeezed lime juice, not a sweet and sour mix. Very tasty and very potent.
A close second is El Patio on Westheimer inside Club No Minor. They used to top the 'ritas with Everclear which lead to some of the craziest happy hours in town... but they stopped topping a couple of years ago. Still serve a great margarita though.