Reshiram...
When I first saw that thing, I thought it was sex. That was the only word for me to describe it. Just a vision of artistic beauty and perfection, realized in a Pokemon. Its design is flawless to me, combining elements of three of my favorite creatures (dragons, canines, and birds) in a stunning way. With a partially mechanical design too. Love the turbine tail and the wispy natural design of it. And it's white! Pure, beautiful white. I love that color. Once I saw it, I knew which version I'd be getting. Didn't matter what the exclusives were as long as I could get my hands on this majestic fiery beast.
My second favorite would have to be Latias. It is female, it is red, and it has lovely yellow eyes. (I have a bit of a thing for yellow eyes.) Latias is basically what I'd be if I were a legendary. And it even comes with a male companion, just like I do. Its sleek, flight-based design is very appealing to me. A beautiful bird dragon plane. And it's Psychic-type, my favorite! Oh, that really adds to it. It also is more relatable than the majesty of box legendaries. It's small, it's cute, and looks like it could easily be a regular Pokemon. This is something I'd consider using even beyond my ban on Legendaries in my team in-game.