torchy's is pretty rockin for tacos. but i prefer authentic mexican when taco hunting. so look for a hole in the wall or a taco stand downtown.
it really really depends on what you like. austin has so many austin-originals you kind of have your pick. casa de luz for vegan. magnolia for queso. ruddy's for bbq.
however, i'm gonna say *P. Terry's* on Lamar and Barton Springs for an hamburger. double cheese is awesome. meat is organic from a local farm. also, the veggie burgers are amazing. they hand make them and i usually get one even tho i'm not veggie only. 9 months out of the year there is a snow cone stand setup next door, get one. :)
This tip has the address "central"