Favourite Shikai Reveal: Originally, Senbonzakura's shikai. Kubo did an excellent job of making it seem beautiful (because of the name) and terrifying. But that was also because I first read an English version of the manga which mistranslated the word for "Scatter" as "Die". The shikai I find most interesting is Kyōka Suigetsu; not because of the overwhelming power of its Kanzen Saimin, but because of its name meaning and all else it connotes. I also don't believe Aizen ever attained bankai, making it even more fascinating. I like Minadzuki because it's silly, takes the form of one of my favourite animals, and Unohana pets it. Engetsu because it responds to Isshin's blood. I am curious about Yoruichi's, but I really, really, truly, do not believe she has the ability to use her zanpakutō anymore. Unless she got it reforged by Ōetsu when we weren't looking and Byakuya is hiding it in a secret compartment in his Ōken clothing. Dubious, to be sure.
Least Favourite Shikai Reveal: Shikai generally bore me so I am indifferent to the rest.
Favourite Bankai Reveal: Senkei: Senbonzakura Kageyoshi/Shūkei: Hakuteiken. Byakuya sheds his own blood to activate Senkei, and it has a story behind it which has yet to be explained (I have read that the English translation of the kanji used for his "Senkei" is "Slaughterscape"; I like it). Ichigo's facial expression upon its activation is golden, too. Hakuteiken's visual design and name meaning possibly evince imperial/empyreal (yes I think I am clever) power. But it's probably emblematical and not a real manifestation. Hakka no Togame, because I shivered. I am holding out for Ichigo's real bankai, and all the currently unrevealed bankai. Also: Minadzuki because it's morbid and it's Unohana and we don't know a lot about it. Komamura's bankai (Dangai Jōe) during his human transformation was a real improvement, and revealed fascinating details about his origins.
Least Favourite Bankai Reveal: I don't really care.
Ulquiorra's entire character arc, and therefore his two Resurrecciones, are worth mentioning. Otherwise I don't much care for the Arrancar as a rule. I will say that I think Kubo was trolling Byakuya with Sèptima Espada.