My favourite? Easily Azelf with a few different reasons. To be completely honest I prefer small and adorable legendaries but still with the power and strength of those who are towering and can probably crush me with one step. I chose Azelf mainly for it's title as the Willpower Pokemon, willpower is an amazing quality with the other Lake Guardian values.
Honestly I liked it for it's simple design, it wasn't too overdone and the colours mixed together well unlike Mespirit's, grey and pink is just an awful combination to be completely honest. Also out of the other small legendaries I disliked Phione for being basically a tiny version of Manaphy and Shaymin wasn't too much to my liking, so Azelf took the cake and claimed a snug spot in my heart. <3