I like Ho-Oh and the Legendary Dogs Cats Gerbils Beasts because of the history behind them. They have a deep history that connects them, and I'm a history freak, so that's what I like.
But Lugia is my absolute favorite. It was the first Johto Pokemon that I liked, and that I liked for many years. The design, the typing, Aeroblast, and origins all roll together to create a Pokemon that I just like.