Crystal Towers 2/YMMV

Crowning Music of Awesome: Pretty much the entire soundtrack. Listen to it here.
Surprisingly Improved Sequel: By a lot.
That One Boss: Beam Stack.
That One Sidequest: Quite a few, especially surprisingly difficult rainbow gem challenges in disproportionately early levels such as Walnut Creek A's "Watch Out for Bees!" and Luminous Tower A's "Get 1000 Points Without Falling." Common consensus is the Dust Hill A "Beat the Ghost!" challenge is the most notorious.