Depends on the burger I'm making.  Lately I've been doing a lot of 
smash burgers.  It works well with 80/85% beef or meatloaf mix (beef/pork/veal).  For those, just copious amounts of sea salt - they taste good enough by themselves that they don't need much else :thumbsup:  If I'm grilling a turkey burger, I do soy sauce & garlic powder.  For bison burgers, sea salt, garlic powder, and onion powder.  Here's one I did last week - combo of sauteed onions & mushrooms with my buddy's awesome homemade guac:
http://i.imgur.com/tBRAntn.jpg
For toppings, depends on the mood.  Recently I've been working on sauteeing, so a lot of carmelized onions with mushrooms, as well as guacamole, tomatoes, ketchup, mayo, etc.  They're finally coming out with some decent gluten-free buns I can eat, so I'm pretty happy about that from a food allergy perspective.  I like Boston lettuce or baby spinach too, instead of iceberg.  Bacon is always a hit, and turkey bacon is pretty decent.