Best Art: Mountain
Yeah, I went with a basic land. So what? It's my list! Mountains nowadays are usually drawn with odd formations, fire and flames, or harsh desolation but that's not what mountains are (I should know since I live in a mountainous area). This art has a peacefulness and tranquility that comes with those giant lumps of stone and dirt we call mountains. I know that betrays what red does, but it's what mountains are (at least to me).
Worst Art: Molten Influence
I don't know what it is about this piece that has me so off-put. The work on the right side is beautiful, but the left side seems so... out of place? It kind of looks like there was a different concept that was almost completed, but then the decision was made at the last minute to say "screw it; it needs more lava". It really doesn't feel like both sides of the card are interacting with one another. It's like the wave of lava is blocking out what the mage is firing at. Just not my favourite art.
Most Thematically Appropriate Art: Insist
What is the art of? A bear breaking through a blue wall. What does the card do? Allows your bears to break through a wall of countermagic. It's very apropos and I love it. I wish more card art was like this.
Franz Vohwinkel is an artist that 99% of Magic players won't recognize if you say his name, but I can bet that everyone will recognize 99% of cards by his art alone (if they were playing during those sets at least). I highly recommend his website for examples of his breadth of work.