Robin got my attention from the very beginning. She was very mysterious, her behaviour was weird, we didn't know what she was planning,... I remember her assaulting the Strawhats in Whisky Peak, and offering them a log to reach Alabasta; also when she saved Luffy from getting swallowed by Crocodile's sands, and asked him about the Will of D; the high bounty she had; the poneglyph event and she lying to Crocodile,... She was still quite mysterious in Skypeia. I felt like we started to know her well in Enies Lobby and her flashback.
What's more, she's not only just a very special character. She's the only one in the world who can read poneglyphs, she's been escaping from justice and everyone for 20 years, she's been with the revolutionary army for 2 years,... Even Dragon likes her, and needs her to exist for some reason we ignore by now.
I think she's a pretty cool character. And she knows a lot about the One Piece universe we don't. Right now, for me, she's the most important character of the series. I mean, Zoro could die and there would be a swordman less, Sanji could die and they would need another chef, but Robin dies and,... Screw all the story. They cannot reach Raftel, they cannot discover what happened in the void century,...
And her design is beautiful. She looks better by far in the manga than in the anime. I still prefer her old design rather than the one she has now. As somebody said above, Ray Down I think, she looks like Hancock with glasses; it happened to me many times to mistake Robin with Hancock fanart. Hancock is the most beautiful woman in the world, and Robin looks like her, so we could conclude that Robin is the second most beautiful woman in One Piece.