I've had a lot of plain and fancy burgers in my time, but as soon as I saw the question, I immediately thought of my top four.
Bill Gray's - Webster NY. It's my hometown, so this will probably always remain my favourite.
CB and Potts - Fort Collins, Colorado. Oh, mama! Not sure if this place still exists, but I spent a lot of time there, while attending CSU. Go Rams!
Ferg Burger - Queenstown, New Zealand. The line of customers usually goes around the block, but the wait is worth it. Fantastic burgers.
Vili's Burgers - Rarotonga, Cook Islands. We normally try to eat as much local food as possible, while visiting the islands, but this is our go-to spot. Can't wait to go back again!