It changes from time to time, but currently I would say Mindlathe. I just really like it.
I do also have a soft spot for Purgatos, made a Magmar deck with him as the star once and when it worked it was rather brutal 
But then, I have soft spots for so many ‘underused’ cards
Bounded Lifeforce
I have no words for this…
Anyway
It’s Doom
when I’m playing against anyone and they don’t get what I’m doing, I sit in the corner with provoked silhouettes and abjudicators, and then suddenly they have 3 turns to burst me down from 25 they know they’re dead
(except mantra and move kaleos players. they suck)
Definately either Blood Taura (as long as it’s played skillfully) or Grandmaster Kraigon.
I don’t know why, but there’s something about Rogue Warden.
I want to know the story behind her.
Obliterate basicly a time bomb and is one of the few cards that has its own and only for itself animation
<3
Zendo, no contest. His effect is just so unique and his art looks amazing
Usefulness: Pax, I almost never replaced it. It’s always like: “Oh sweet, my opponent has to deal with that 2 drop or he’s one Inner Oasis from an annoying board away;)”
Art: Maybe a bit strange but I love the art of winter’s wake. I just imagine the snowy, silent forest that get’s illuminated by a powerful spell…a shame that the cast animation isn’t so epic
Rook, he may be slow and he may be weak, but damn is he funny to play
Onix jaguar for the win.
And thunderhorn:)
it use to be chrysalis burst…now I just don’t know.
Monster Reborn
Grandmaster Zendo, if you want to be anal about it.
Portal Guardian, he just looks so damned cool! And again, geokinesis
Anal is defined as someone who is really uptight or particular and who always wants things a certain way.
Read more at http://www.yourdictionary.com/anal#JLAw3WFK9XOG2A6w.99
So yeah, it has multiple meanings.
Desu-desulator 
rogue warden was exiled from the lyonar
some say it was for love
others say it was for money
but they can all agree that she was better than solpiercer
personally i think it was because she dyed her hair red
my favorite sprite is ruby rifter
but my favorite card to play is variax
because they’re practically kamen riders:
Abyssian rider
ruby rifter
because the animation is dope:
paddo
pandora
sunsteel defender
