My favorite Day is the one where you work to go to the beach, followed by the tournament. My least favorite is the Seven Wonders. The characters getting pissed off at Roxas for wanting to know the 7th wonder at the end always irritated me, and then you have that really long scene with Namine that's prolongued too much by empty air.
From best to worst, keeping my explanations simple:
Day 4: I love the struggle tourney in its entirety.
Day 6: Several really memorable moments take place here.
Day 3: I almost put this above Day 6 for the fact that I love returning to the Dive to the Heart, as it is such a perfect tone piece for the series, and ties Roxas to Sora so well early on. I'd love to see Dive to the Heart return in KH3 in some form, however likely or unlikely that may be (separate discussion for a separate thread).
Day 1: Somewhat unspectacular, but provides a decent set up for the prologue and raises a couple good questions.
Day 2: The job minigames aren't terrible, and this day can be skipped for the most part, but if you choose to do them (and get that small little AP bonus) it just winds up being tedious. It isn't horrible, but it's not too great either.
Day 5: The most obscure, and the only one I'm not a fan of in any form. I found a couple of the minigames here tough when I first played the game as a young kid, and now, much like those of Day 2, they feel a bit tedious and unnecessary, except you can't just skip it and have your friends do all the work for you. The narrative weight of this day is arguably the least of all in the prologue. One could make a case for Day 2 being worse than 5, in my opinion.