Merlin, this is a tricky one. I try not to be partial to magical creatures because they're all so different, and there is something unique and wonderful about each one. But I think I can narrow it down for you (in no particular order).
1) Thestral: They are rather depressing creatures, but they are so mysterious and lovely. They tend to be gentle unless provoked. I always enjoyed seeing them at Hogwarts.
2) Caladrius: This is a gorgeous white bird. It's similar to a Phoenix in a way. It sucks out a person's injuries and shoots them into the sky! Neat, right?
3) Baku: A Baku eats away bad dreams before they turn into nightmares. I don't think I've ever experienced one at work, but they seem really interesting.
That was hard, but there you have it!