The fights in the Mukuro arc made me think that Amano Akira was kind of testing the waters and seeing if his/her series could really survive and maintain people's interest if s/he were to switch from pure comedy to action. None of the fights were really as amazing as the Varia arc fights, in my opinion. Really, Mukuro's arc was kind of an 'action oriented daily life' set of chapters- the main purpose seemed to be to introduce action and, of course, Tsuna's future gaurdian of the mist, Mukuro (well, I guess only the person that helps his guardian, technically, as Chrome would be the Mist guardian).
The Varia arc had all the coolest fights, but, honestly, I thought the plot was kind of lame up until the revelation with Gola Mosca and 9th. And Xanxus vs. Tsuna had a very anticlimatic ending to me.
I think the Future arc is probably my favorite arc right now. So far, the action hasn't been that far behind its predecessor, and the plot and all the intricacies of it are much more intriguing than seeing Tsuna's family square off over a set or rings, just basically because they were told to by Reborn/Iemitsu. Not only does the family have a reason to fight, they don't have a choice as they're trying to be eliminated.