American Magic: The Gathering artist Noah Bradley recently revealed his ranking of how easy/good the basic lands of Magic are to paint. Over on his Twitter account, Bradley started a thread explaining how easy or hard it is for him to paint each basic land. He said that Forest is the easiest to paint while Swamp is the hardest. Here's the order of his rank from easiest to hardest: "Forest > Mountain > Island > Plains > Swamp"
"Forests are easy. Lots of color variety. Cool shapes," Bradley said. "Not a lot of difficulty. Trees are the shit."
As for Mountains, he said: "Mountains are also easy. Paint something kinda rocky and boom--you've got yourself a mountain. Just don't paint too much water and you've got nothing to worry about here."
While Forests and Mountains are easy for him to paint, he said that Islands start to get tricky. "Are you painting an island or just a coastline? That's a tough question," he said. "Generally pretty easy, they're kinda tough because you start to have to paint an ocean and I've always found that difficult."
Bradley went on to explain why Plains are very hard to paint: "Having vertical elements in a landscape make it a lot easier to paint. But you can't have too much of that in a plains. Can't have big mountains or forests or anything."
Finally, Bradley explains why it's the most difficult to paint Swamps out of all MTG basic lands. He says that adding certain details of nature could make Swamps also look like other basic lands.
Too many roots/trees? Oops it's a forest. Too much swampy water? Shit it's an island now. Put a mountain in there for something vertical? Nope. So yeah, swamps are the hardest. It's official.
So there you go: Forests are the easiest to paint; Swamps are the hardest, and I'm sure most MTG artists would agree with this rank.