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.