My favorite is by far Raikou. Loved it since childhood. Probably due to having gotten from a friend a very cheat Shiny Raikou on White. Yeah it's cheat I know. But he's just so beautiful! And his shiny is my absolutely favorite among all shinies
I'm definitely a Suicune person, if you couldn't already tell by my profile. Lunala would be a close second for me, I just love it's design. Another one that I like is Virizion. I've had a Virizion named Spiral for as long as I could remember. It was one of my first legendaries and I was overjoyed to have finally caught it.
Others that I like are Ho-oh, Reshiram, Raikou, Dialga, Ghiritina, Yveltal, Cresselia, and Latias. Basically all of them haha. It's hard for me to choose because I personally think that (most of) the legendaries have awesome designs. Oh yes, and I don't know if mythical pokemon are included in this discussion but I do love Arceus. It was my favorite pokemon at one point.