(Just my ignant-ass opinion.)
The OP assumes dude you're fighting has no friends. BJJ sounds great until you start getting stomped by dude's homeboy. Lotta places I wouldn't want to roll (streets with broken glass and shyt on em, etc).
Boxing/MT/Kickboxing are all great, assuming you have space and your opponent can't fight either.
@FabTrey hit the nail on the head, what if the CAC you just ethered was on that Iowa wrestling team? Boxing may not help you if he takes you down before you can knock him out. If the dude you're fighting is in a higher weight class and has hands, you might be fukked.
Gun-fu sounds great too until you catch a murder or attempted murder case.
Best bet is to either train in MMA/Martial arts used frequently in MMA (wrestling, sambo, bjj, mt, boxing, kickboxing, judo), be skilled in them, and never get caught lacking without a tool or being by yourself.