This is where the fun happens, imo.
Like other healing specs, being good as Holy depends on adapting to the requirements of the encounter. It naturally follows, then, that fights that allow Holy priests to dynamically adjust their Chakra to their precise role in the encounter at that moment ... are the most fun.
There were some of these fights in Firelands:
Archdruid: scorpion phase as Serendipity; cat phase as Serenity
Ragnoros: phases 1 and 3 were very effectively done in Serenity; phase 2 (and 4) as Serendipity.
Getting involved in the decision about how and when to deploy Chakra is a large part of the fun for me, so the fights I like best are those that allow me to exploit different Chakras at different times. There weren't really many of these fights in the last tier of Cataclysm; they were all pretty much AoE spams. But, I'm happy to say that I found Blizzard's fight design for the first tier in MoP to be good fun.
Tsulong asked you to be in Serendipity in his night phase, but the day phase was the perfect occasion to use Serenity.
Garalon really needed to have you in Serendipity with the dps, but when you went to kite pheromones, switching to Serenity really helped to deal with the self healing and damage.
Empress was addressed with Chakra: Serendipity for the sonic disruptions and the last phase, but during retreats when those hard hitting adds came out, Chakra: Serenity was so much more valuable.
I love fights like this which add this special level of involvement for Holy priests .. and so, for those, I would like to say, thank you Blizzard.