What game would you nominate for this honour?

I'm not sure, but probably Ori and the Blind Forest.

Rayman Legends. Gorgeous looking game, fantastic soundtrack, fantastic gameplay and level design. I think I beat this game like 6 or 5 times lol.

This is one of the two games I was thinking of when I clicked into the thread.

This is the other game I was thinking of.  

Both are beautiful games.  As to why I think they're beautiful.... how does one answer that question?  They just *are*.  

Came in here to say both of those games as well. I would say Ori is the most beautiful in the traditional sense. Rayman Origins is gorgeous if we go for the nontraditional sense. 

Ori is the first game to come to mind.

Also came here to say Ori and the Blind Forest.

And I suspect you made this thread to see if Muramasa: The Demon Blade would get any mention...

Do 2.5D games count or are we talking specifically 2D? With Bloodstained being so fresh I can't help but think of it, it looks so good.

If it's strictly 2D, though, I suppose I'm going with Ori and the Blind Forest too. I'd name a couple honorable mentions, but when I start thinking of them it's too many, so, I won't.

I'm gonna say Odin Sphere because that game was awesome and the art style is what made me fall in love with it.

You can include 2.5D if you like.