Del Fagioli is one of my favorites. I never got bistecca there, though. Maybe next time.

Two specifically for bistecca alla fiorentina that I really like are osteria dell'enoteca and trattoria mario. Not to be confused with buca mario, which is kind of a tourist trap, except that it's actually good - its just overpriced and really busy.
For traditional/classics - trattoria sergio gozzi. Go early or you're not eating there that day. Open only for lunch. La casalinga is also great if you prefer to be in oltrarno, and is also open for dinner, but I'd still call/email for a reservation the day before or the morning of - you can walk in, but you'll often wait.