My top favorite is Latias. :3
I have good memories catching Ho-oh and Latias. Okay so... I was traveling with my family to the mountains. I remember being at a rest stop area. I had my trusty gameboy color with me. I was on the top floor of Tin Tower and there was the wonderful Ho-oh. I named it Howard after my dad's middle name. Well, I formed a habit of calling a Pokemon some name starting with their first letter (in this case, H). As for Latias, I was surfing west of Pacifidlog looking for the area to dive to start the Regi golem mission. I ran into Latias on the water. I chased her around for about an hour until I caught her using a Timer Ball (long battle!) on the route above Slateport. I named her Lilac!... And I caught many more Latias as time went on.
Mostly, I just love their designs in general and/or their representation in the anime. Except Arceus. His movie was just silly. He's still cool though! XD;