Why not? It was a huge improvement over its predecessor. 

OT: I thought about picking MK, but I'm actually gonna go with Xenoblade.

Eh, the graphics themselves are ugly. It's got a few nice touches here and there but it really isn't a looker like other Wii U games. Sure there are good examples of the game (I dont need screenshots to know) but overall I feel its a really weak contender compared to MK8 or Pikmin 3 as examples. It, like its predecessor, are a bit muddy looking. But if you like it, that's fine. But graphically, to me, I think XCX is an ugly looking game, XC itself wasn't pretty either. SM3DW was also a nice transition

This thread isn't about which looks the prettiest though, it's about which represents the most impressive leap over its predecessor. Not necessarily the same thing.

For example, 3D World is very pretty, but Mario Galaxy already looked so good that the leap isn't that impressive, in my opinion anyway.

I like how 3D World looks over Galaxy, gameplay is a different monster x.x XD But in overall feeling I do believe MK8 has proven thus far to be the best HD game from Nintendo. But I see Zelda U taking that crown

If not talking about graphics leap only, I definitely go with Zelda U. Game looks so good, and world looks so huge, beautiful and alive. Skyward Sword looks nice, but it was very linear and world wasn't big.

Well no it is mostly about graphics.  But its about the leap, not just the beauty of the HD game.  What he meant is 3D World is a very pretty HD game, at times gorgeous.  But Super Mario Galaxy is a game that holds up remarkably well and thus the gap between Galaxy and 3D World is not nearly as big as Mario Kart Wii and Mario Kart 8, for example.

Exactly, this is the reason, like I said before, why I find it a bit strange that people are picking MK over Pikmin...but, opinions and all that...;)