Arcee for me. Just Arcee in general, but if I had to be specific, TFA version.
Summer of 2010 I worked at a camp and went by the name Arcee. I carried the TFA toy on my lanyard every day and the kids loved it. The played with her, puzzled over how to transform her and her existence let them know I was a TF fan myself, which helped some of them to open up quicker. Not to mention she lead to many a fun and geektastic conversation.
I had always liked Arcee, but that summer made me adore her. It spawned a cosplay several months later actually. haha. Too bad I didn't have it for camp, but kids I run into when dressed up love it xD I've had several come right up to me and ask for pictures (ie: the TF American Tour, at which I was the only dork dressed up and loved every moment of it xD NYCC had a few too). I think the best was when I'd walk into their line of sight and all of a sudden they'd yell "MOM! IT'S A TRANSFORMER!!" So loud, but so unbelievably awesome xD
(no, I'm not the one who has the full mask and yada yada, if you've seen her cosplay. Haha. I'm kind of a human hybrid of the TFA Arcee)