Mine was a tie between Dragonite, Goodra, and Kommo-o
Dragonite because it holds sentimental value as the OG pseudo legendary. A big dragon with small wings just gave it a fantasy sort of appearance, not to mention that my sister had a Dragonite card that I would try to use as a reference to draw all the time. I really like its color scheme, more so towards the earlier sprites because I felt like they had a lot of character to them, but I still love Dragonite now.
Goodra just has this sense of adorableness both in design and pokedex entry. It loves hugs, its got this smooth design, the colors are solid, and I happened to use one on my X playthrough, so I got attached to that one as well. I just loved going into Pokemon Amie and seeing that sweet smiling face. Seemed like an all around great Pokemon to me.
Kommo-o is interesting because it's the only one here I haven't used on an actual team, but I place that mainly being to how late you get it rather than a testament to how good or bad it is in battle. Kommo-o has an awesome design and learning about the real life inspiration was cool as well. It gets Bulletproof as an ability, which is fantastic, and it has this dinosaur look, which I love.
It's funny, the only pseudo legendary I don't actively like is Garchomp and not for a lack of trying. I tried using it on a Platinum team, but it didn't really do it for me. I absolutely love sharks, so it should've been a slam dunk, but I don't know if it's just been overhyped to me or that I've seen it everywhere since Gen 4 launched, but it's just meh to me. Doesn't mean it's a bad Pokemon, just not for me, I guess.