This is the most played game in my library? Guess that speaks for itself, then. And it still keeps updating with new features, it's amazing.

I guess it's actually not #1 anymore, but it's still #2 and I'm well overdue to take another crack at it.

EDIT: In December 2019 it's still #2, woo!
EDIT2: In 2020 Steam doesn't want to tell me what my most played game is, but Terraria is better than it ever was!