For me it's hard to pick an all time favourite regarding the artworks and stuff like that,because I think there are loads of great card designs.
When looking at the gameplay and abilities, I tend to have new favourites with every set, since I mostly play modern and the meta there keeps changing.
Currently Kaylia Silverleaf is my favourite Archon. The -1 magic attack is great in many battles, since magic attack is very strong in general so it's good to counter it a little bit. In addition to that, I really like the Bloodlines mechanic and the execute ability can turn many cards into absolute beasts.
There is certainly plenty of awesome cards to choose from and more yet to come. Thanks for sharing your current favorites with us. Magic has always been a beast in game and having a card that hands out -1 magic attack is a good choice. All the best on the modern battle-field.