I voted for Cap vs Loki, although if you are gonna bill the next fight as Thor vs Iron-Man/Cap, you should have billed this one as Cap/IM vs Loki, because when IM swoops in to the strains of AC/Dc it is one of those heart warming hero team up moments that the movie needed in order to work, there was more of this to come of course, but that was the best one, as it was the first meet up of the core Avengers onscreen, and because Cap was fighting a battle he could not have won.
But, yes, the main fight is between Cap and Loki, and what makes me like it most is the fact that he is , essentially, a guy with no *real* superpowers fighting an extremely powerful 'god', and doing his best to hold his own until the calvery arrives/people can get to safety.
The enterance scenario for Cap is perfect, and the way Chris Evans plays it is just right as well, like the character of CA on the whole, it could so easily slip into cheesiness, but instead is good old fashioned no-nonsense earnest heroics. 'I'm not the one who's out of time.' How easily could that have been cheesey? but somehow, almost magically, through the right degree of acting and directing and context, is played with the aforementioned qualities.
Sometimes it is good to see a hero onscreen who is not taking the piss all the time, lol, as they usually do with the one liners, that was a one liner, but it was more a statement of intent, just telling loki he was there to do the job.
I have watched that whole sequence, played through to the IM vs Thor fight, many many times. It is perfect superhero cinema.