I like Rarity because she is, for the most part, just a really
good person. She really does want to help her friends, and I mean beyond stuff like "Suited For Success".
I mean, even in "Sonic Rainboom", she was the first one to notice how nerve-wracked Rainbow was, and took charge in finding them a way to go to Cloudsdale to support her. She went so far as to be the guinea pig for a wing-spell that Twilight had never even attempted before, and said could be tricky to get right. How funny that the one who most wanted to help Rainbow was the one who lost track of her original mission so easily (but, to be fair, those were
dangerously fabulous wings.
Or "Green isn't your color". Yes, she was extremely jealous and resentful of what was going on, but she
knew that she was being that way, and resented it, even though she couldn't stop herself from feeling that way. In spite of her jealousy, she kept herself from telling Fluttershy that, so that she could let her keep doing what she was sure would make her happy. There are quite a few shows where someone in that situation wouldn't be that self-conscious until the end of the episode/aesop moment.
Yes, she does have her faults, but she genuinely tries to be a nice person and, well, I think she does a pretty darned good job of it.
I have a message from another time...