if you could master any martial art...

For me personally I wouldn't sit still in just a single martial arts. I'd probably perfect my Karate but the few martial arts I'd really like to master would be Wing Chun, Kung Fu, Muay Thai and Taichi. :)

Wing Chun, Krav, and of course Systema. Putin would be piss if I can't defend against hooligan hoards in Moskva.

If I could master a martial art, it would be the "Invincible Martial Arts Style" or something, that way, I am a master of an unbeatable art, and would be able to defeat any attackers, even other martial arts masters, with relative ease.

Probably capoeira or muay thai when it comes to unarmed combat.
I did some taekwondo when i was younger but i did not like it that much.
(And i got kicked in the back.. Helloo~~ back problems)

When it comes to armed combat i'd love to learn a spear or staff style of some sort :)

I'd master northern Shaolin so I can impress people with my flying kicks. As for armed, probably knife-fighting.

It would have to be Capoeira. It just seems like the one that would be the most fun to me.

Systema or MMA ausemastering themmeans you'll be fit as hell.

Systema or MMA

I would like to master Tae Kwon Do, Karate, and Kung Fu, so I could make a fusion of the 3.

Anything that's flashy. Ninjutsu would be nice.

I'd just finish my thai training.

I've been doing Taekwondo for 5 years and it'd be nice if I mastered it.

Shaolin disciplines be pretty bad ass....There's some real wall running in that shit

I wouldn't mind trying out Aikido, Ju-Jitsu and Wing Chun.
Caperioa and Jail House Rock seem interesting.

Kyokushin Fucking Karate

If I could master ANY martial art, I'd master ALL of them. After all, "All martial arts under the heavens" is a valid 'martial art', but none have ever 'mastered' it.