Based on my personal experience, I doubt there is one single discipline that can cover all combat situations. I think mixed martial arts disciplines are your best bet, and more precisely disciplines that cover both your striking and ground game.
For striking I think going for kickboxing or Muay Thai is great because both teach you to be tough as nails (when I train with some friends we warm up by kicking each other in the legs, for instance) and focus on gross motor movements which are easy to commit to memory. You can be effective in months of training instead of years like Kung Fu, which is much more form-focused and situational, and requires a lot of finesse.
For ground game, grappling and wrestling is great because you learn how to take people down quickly. Even beginner wrestlers with a basic foundation will do wonders against a striker who has zero ground experience. Once down, ground and pound handily works against even experienced strikers who don't know how to defend themselves on the ground, and you don't really need to learn much in ground and pound; again, it's mainly gross motor movements that are easy to commit to memory.
Of course, if you have more time to master it, Brazilian Jiu Jitsu is great for locks and submissions.
I get together with some buddies every Sunday and we exchange training routines and techniques: one guy is a Pan Asian games silver medalist in Kyokushin Karate, another is a Muay Thai instructor, and one guy is an absolute beast that knows many disciplines throughout his 31-year career as a MMA instructor, including BJJ, kickboxing and Aikido.
Then there's me with my Kung Fu. I can honestly say that the stuff I've picked up from them - especially Brazilian Jiu Jitsu and Muay Thai - has helped me much more than my Kung Fu has in combat, and I notice this because when we free fight in my Kung Fu class I employ a lot of their teachings and even experienced black belts find me a handful (I'm a brown belt now but this was true even when I was just a green belt).