Hello there and welcome to the Pokemasters.
If I had to pick one favourite Legendary Pokemon, it would probably be Articuno. The first time I caught him was in 2000. I think it was in Pokemon Red. It was winter and on the same day, I had had to walk several kilometers to a shop to buy some bread although it was like minus 40 degrees below zero.
Anyways, to be honest, I'm not that found to use Legendaries in battles, at least not anymore. I enjoy the challenge, as the Pokemon series are games that offer a lot of variation to play through.