Smooze and haircuts for me. Also marker stains and stains in general, because I find them impossible to get out.
Smooze has effected Medley, my first pony, for over 32 years now. (I've had her almost as long as I've been alive, minus 3 years, plus whatever year she was made) I've tried getting it off, but it's been in there so long, it looks to be permenant. Sad and frustrating, because I wanted to get her a new yellow tail ribbon once she was all cleaned up, but that was before I knew how difficult smooze was to remove. I'll probably get it for her anyway, just to make her look pretty.
I don't usually buy ponies with haircuts, but if I did, it would be tempting to try and rehair them.
Edit: Wow, Medley's hair is really dry looking too! (Took her off the shelf to hang with me.) Well, that's on the list. (Haven't figured out how to fix it either. Used conditioner, it didn't help, obviously.)