Best Burger I've had from a chain is most definitely Red Robin. But assuming Red Robin isn't "fast food", I'd go Freddie's Burgers number 1, followed by Shake Shack, Five Guys, In and Out, and Culvers. In and Out is really good, especially to be as cheap as they are, but I thought they were a bit overrated given their reputation. Used to love Streak and Shake, but feel like they've gone downhill the last few years.
No other chain fast food I've tried is really even on the list for me. McDonald's, Wendy's, BK, etc, are all pretty much trash. I'll eat Wendy's occasionally, but it's been awhile.