I'm not a fan of the bling burgers - for me, it's all about the meat and some decent cheese. No pickle or sauce for me - get the basic flavours right, and you don't need all the rest.
A couple stand out:
A farm stall at a weekly market at Woburn, Bedfordshire (everything was just right - the top-quality, slightly rare beef, the freshly baked roll, the sharp, crumbly cheese...)
My home-made ones! Most recently, barbecued on the roof of a narrowboat somewhere in Cheshire. Mmm. Plenty onion and chilli in there...