Mine is definitely Necrozma. It's probably one of the most interesting Pokemon designs ever, and I like how it is kind of it's own thing (not involved with the Ultra Beasts, Tapus, or any of the other Legendaries). Necrozma-GX is also a super fun card to mess around with.
Xerneas has a kickass design and is a force of nature in competitive battles, so I have to go with it. Lunala is a close second; it also has a great design and is an excellent Choice user.
Lugia for me, ever since I got Pokemon Silver when I was little, and of course, from the second Pokemon movie. It just has an awesome design, I just wish that it was a little better competitively, when I tried to build around it, just always felt a little lackluster.
I think my no.1 favourite is Kyogre; I've always been drawn to sea life and have always had a thing for giant sea creatures. Plus I love the lore behind it (and the Weather Trio as a whole), not to mention I adore its Primal form.
Honourable mentions include Hoopa (Confined form), Xerneas, Lugia and Moltres.