Spike and Applejack are massively underrated. Then again, you have to have a certain level of maturity to properly analyse and understand their character, looking past their flaws and "boringness." Boring is a bit inappropriately used to describe these two, as AJ's the most hard core pony in the entire show, and Spike's a freaking dragon.
Also, Princess Cadence and Countess Coloratura deserve a lot more love, especially which such prominent but quick character development.