They're not everyone's cup of tea, but I'm personally very partial to white castle. Those onions just do it for me.

Out of the big players, in general I like Wendy's, but they almost fall into an uncanny valley sort of thing where they're so close to being a real (not fast food) burger, but not quite there, so I kind of have to be in the mood.

IMO burger king has the best veggie option in the impossible Whopper.

If we're going to count it as fast food, 5 guys is top notch. Similarly for shake shack, they also get bonus points for their shrooms burgers for the best veggie option overall, it doesn't try to be a regular burger, and is probably actually more unhealthy than one too.

Getting into weirder local chains, Cook Out is pretty great if you find yourself in the parts of the south that have them.

If we want to talk chicken sandwiches, Popeyes is the undisputed king and I will fight anyone who says otherwise. McDonald's crispy chicken gets honorable mention for being a solid alternative for people like me who refuse to eat at chick fil a, and the McChicken for being cheap and satisfying if not technically good.