I just came back from 6 days in Chicago.... I knew in advance that I wanted a Chicago style hot dog. Two people - prior to my trip, and one person while on my trip - told me to never ask for ketchup on a hot dog. Now, I would not do that as I prefer mustard, but if I asked for ketchup and the vendor said "no", I would just walk away.
That said, I had Chicago style hot dogs in three places last week..... 2 were disgusting. One was AWESOME.
Now, I plan to have a party at my house featuring hot dogs from various parts of the USA. There are dozens of styles from all the regions.
Anyway, what goes on your "perfect" hot dog?
I like mustard, onions, relish..... and occasionally a bit of ketchup. Sometimes bacon, but if I do that, no relish.