Far from "the best" restaurant in Boston; but a good and fun spot is Fajitas and Ritas just a block off the Commons on West st. Way better than average Tex Mex and killer margaritas. Casual and cheap for the quality.
To HuskyHawk's note about not venturing out of his region, herein lies the issue for the folks in the burbs and the above average but not spectacular restaurant in Boston. If I'm going to Boston, I don't want to pay to park or take the T just to hit up an above average restaurant. It needs to be because we're going to the MOS, aquarium, sporting event, chinatown, park or a special night out. So I usually miss out on a solid restaurant like this and instead end up at the Fours or pho. Even the days of going to Dim Sum in Chinatown have changed - in the south there's Chau Chow and north of the city, places like Ming's Seafood in Malden. I guess it's a good problem - as the population has spread, there are lots of great options for us throughout the metro area.