Rarity, she is the one who suffers most from her element. Loyalty is something we all have, that's why traitors are looked down at. It's easy to joke. Honesty is expected from all of us. Magic is a gift. Kindness doesn't cost you a cent.
But generosity? Yeah that hurts. Every time you give something away, you have less for yourself. Which wouldn't be a problem in short term, just give what you can spare. But to be forced to give? To give everything you have away, not out of goodwill, but because it is expected from you? The others pay a very petty cost for their elements, Rarity pays with her sweat and money. She deserves it more than any other.
Do not get me wrong, I dislike her character, I feel she is overbearing, obnoxious and too willing to see fault in others, but is blind to her own flaws. I can see her appeal though.