I've had Five Guys, In-n-Out, and The Habit. Of the three, I prefer The Habit; the best beef of the three, and the most substantial burgers (while still being generally fast food size and price.) Not quite as broad a condiment range as Five Guys, but a good bit more interesting than the very limited menu at In-n-Out. Prices are higher than In-n-Out, but cheaper than 5 Guys (at least around here.)

The one near me is also rarely as busy as the Five Guys near me or even the least-busy of the In-n-Out locations

The only thing I prefer Five Guys for is that they usually (always?) have a Coke Freestyle machine, and the fries are both good and plentiful.

I haven't had Shake Shack to compare, but friends from the East Coast love it.