Kusakabe Maron to Kaitou Jeanne ( Kamikaze Kaitou Jeanne )
Whee. With Fin's power-up, Maron jumps off a building (or ferris wheel or wherever) and as she's freefalling she transforms herself into Kamikaze Kaitou Jeanne, the reincarnation of Jeanne D'Arc. Logically I have a few problems with this - can you imagine Fin trying to teach Maron to do this kind of transformation in the first place? Talk about a leap of faith. (Or hey, maybe they practiced by jumping off the jungle gym.) The anime isn't all that, and the second season takes a big nosedive into mediocre-ness, but I still like first season's energetic transformation.
Haneoka Meimi to Kaitou Saint Tail ( Kaitou Saint Tail )
I'll always have a soft spot for anything Saint Tail, I think. Her transformation sequence is out of sorts since it's always recycled footage - at the end it always has her standing in front of the cityscape, which doesn't fit, particularly when she's transforming inside the chapel. Still, I like Meimi's henshin phrases ("Shu yo tane mo shikake mo naikoto wo yurushi kudasai" and "One, two, three!") and her voice actor's way of saying them, and I love her kaitou outfit. ;) I love episodes 27 and 30, which are both straight from the manga, because there Meimi has a very original way of beginning her transformation.
Tsukino Usagi to Sailor Moon ( Bishoujo Senshi Sailor Moon R )
I actually started watching Sailor Moon (English dub, alas) in the middle of the second arc of the R season, so this was the first henshin sequence of hers I saw - and I also happened to really like it. I admit I also liked the dubbed DiC background music that went with it, which just gave it a unique flavor. Of course, I can't watch the dub without wincing now, but this transformation sequence is still cool to me, somehow. It remains my favorite because this is before I got the urge to start fastforwarding through transformations sequences - there are just too many!
Setsuna/Haruka/Michiru to Pluto/Uranus/Neptune ( Bishoujo Senshi Sailor Moon S / Sailor Stars )
The outer senshi just have the _coolest_ background music in the world. It is awesome! And their transformation sequences match the sophistication and power of the music. Seeing as you only get to see Pluto, Uranus, and Neptune transform in S and Stars (they're absent from SuperS, except for the special/movie), it's no wonder I never tire of their sequences. ;) Click the following for screen caps of the transformations by Neptune and Pluto.
Miho to LaLa ( Fancy Lala)
Miho probably has the most unique transformation sequence. She's pretty creative in drawing her own clothes to transform into (the non-recycled footage is one thing I like). The originality of it is just about it though, because Fancy Lala the show hasn't really hooked me.
Nonohara (Jiyu) Jubei-chan to Yagyu Jubei ( Jubei-chan )
Jubei-chan herself didn't really endear herself to me, but for whatever reason, I really like the series' ending theme (Forever) which is also part of the background music when she transforms. As soon as Jubei straps on that lovely eyepatch, you know she's going to transform into a butt-kicking ninja. And she makes a very cool-looking female ninja at that (though that big bow tied at her back always makes me laugh - ribbons apparently can always be used to express femininity.)
Ahiru to Princess Tutu ( Princess Tutu )
Ahiru is a clumsy and not entirely graceful ballet student, so it's nice that she gets a chance to be the prima donna as Princess Tutu. I like the whole egg theme of this transformation (she's wrapped into an egg, then bursts out of it as Princess Tutu), and it gets high marks simply because the show is my favorite of 2002. ^^
Transformation phrases credited to Hitoshi Doi
All screencaps were taken by me. @.o;