Xufu is my favorite celestial. I rarely use him in pvp, but he's on my capture team with the turnip. With the 100% hit of bite and the healing ability of feed, he's just a solid pet that can take down the opponents, and with vengeance, if my turnip fails against whatever I'm trying to capture, I can hit for just the amount I was hit for last.
I do have him on a pvp moonfire team, along with a nether faerie dragon and a fungal abomination, but that team suffers vs other weather changers and hasn't been a top performer for me. Plus, mechanicals are still popular because of the direhorn, kovok, and death adder lashback. He doesn't do all that well vs mechanicals
I'd put chi-chi as the number two celestial pet, but I haven't used the others at all.