I love Rarity because she taught me that I shouldn't be afraid to shine out and be myself!
She shows her element of generosity a lot, and not just towards her friends; You guys already know what she's done for her friends and all, but you question what she does on a regular basis. For example, even after she found out that Suuri had stolen her fabric, and she knew Coco Pommel had been with Suuri, she still let her come down and talk with her, not showing anger towards her at all. She gives her clients exactly what they want, and is nice to her aqquaintances. (or however you spell that XD)
I think the reason why she can be a little rude to Sweetie Belle is because, well, that's her sister! If you have a little sibling (I don't, but I know how it goes XD), aren't you rude to them even if you're nice to others?
Rarity is best pony, yay!