When I used to do "build your own burger" at Varsity back in the day, used to be beef chilli, onion rings and bacon, but that was a sloppy mess. These days when we do a Five Guys run I'll tend to go fried onions, bacon, jalopenos and barbecue sauce. My favourite meals are ones that nail meaty, sweet and spicy all at once, so that burger does me well.
Although I won't lie, if we go to GBK I'll usually ignore the beef altogether (like a massive gaylord) and go for a Salvador (pork/chorizo patty, avocado, crispy onions, garlic mayo) or my most recent favourite, the Satay (panko crumbed chicken, satay sauce, crispy and pickled onions, garlic mayo, rocket).
I'm more of a dog man anyway.