for the first question, easily the one whose trained. Martial Arts are, well, it's in the name, "arts". being good at one is really complicated. It's probably easier to think of it as an artist drawing something. some people are naturally more skilled at drawing than others, but if you have someone whose trained in drawing most of their life then they'd almost certainly draw the better picture. there's simply too much to it for someone to know it all instinctively. Now, if one was trained formally (like, in a dojo or something) and the other never had formal training, but had experience (like, they just got in a lot of fights growing up), you've got a whole different question. I can try to answer that one too (ask if you're interested) but this post is going to be long enough as it is
Hmm...interesting... I don't think "fighting style" is all that important assuming skill is equal. all fighting styles have their strengths and weaknesses, and a good fighter knows how to compensate for the weaknesses of his/her style with its strengths. the other stuff you listed is all important (though I'd personally think of "stamina" and "defense" as one thing, "endurance"), I'd say the most important thing would be "thinking speed" when defined as "the ability to predict your opponent's moves". If you can put your opponent into an unwinnable situation by understanding how they fight, that immediately wins over any gap in your other stats.
for your last one, I'd chose to specialize in that prediction ability, not only because I think it's the most important, but because I really, really suck at it... technique I've got (well, at least more than the average person). speed, strength, stamina, yep. I've even got quasi-decent fighting instinct, which helps compensate for my lack of predictive ability pretty well, but it's like I said for question #1, knowledge beats instinct every time. If someone can predict my moves and I can't predict theirs, I'm screwed.
PS: just out of interest I'm going to put my background in combat here, and I'm curious about you guys so hopefully I'm not the only one who does this
. I've trained in hand-to-hand combat for almost half my life. I'm a second degree black belt in tae-kwon-do and a blue belt in jujitsu. I've also got about a year to a year-and-a-half experience in parkour (the martial art of running away), but not a lot of formal training, and about half a year training in HEMA (split between the german longsword and rapier).