Well, here are my favorite Legendary Pokemon.
MEW :
Mew has always been one of my favorite Pokemon, since it's so cute and simplistic in design. Though, it's also because of the "mystery" surrounded by the Pokemon when it first came out. That, and unlike many of the other Legendary Pokemon, its design really isn't leaning towards masculinity or femininity; meaning that more people can relate to the Pokemon and project their own thoughts and opinions on the creature a lot easier.
CELEBI :
Unlike Mew, Celebi definitely has more of a feminine design. I believe Celebi is also a lot more wide-eyed and curious about their surroundings and embraces nature a lot more. Celebi can also be startled very easily, while Mew is more laid back. My first exposure to this Pokemon was when I watched the movie,
Celebi - Voice of the Forest. Their quirky attitude and wonderment at everything was truly nice to watch.
LATIAS :
Latias used to be my all time favorite Legendary Pokemon, though it's now taken a backseat as the second or third favorite, trading spots with Celebi from time to time. Just like Celebi, Latias has more of a feminine design, probably because Latios was designed to be more masculine. Latias was one of the first Legendary Pokemon I had ever caught and I always loved their design. Latias is a lot more gentle and nurturing compared to Celebi and Mew; as shown in the movie they appeared in,
Pokemon Heroes.