My current favorite one is a "local" (it's available nationwide though) one called Vicio, which means addiction in Spanish, I tried them because they opened a store near me and had a sign saying "We're called Vicio, and we cook burgers, you can imagine the rest" which I thought was pretentious, and decided to test them, but they're really good burgers.

Before those I would have said my favorite was Five Guys ones, still do but haven't had them in a while. And before those I used to like a local gourmet burger place back in Brazil.