Wii U

Switch 

The improvements are massive, from an upscale 3ds KIU model to a new model in Smash Ultimate.  

Honorable mentions 

For some reasons DK Ultimate version looks worse than Wii U version.

im not a fan of Bowser's design. he is supposed to be yellow. find any picture of him from official Nintendo sources and he is yellow. yet here is some off-color tan, arguably even beige, and it just doesnt look appealing

I think it's more of the lighting in this case. Here's a normal render

And trust me, with a rendering engine like this, it would look awful to just make his skin a classic yellow, and make it over saturated. I know this isn't an in game render, but chances are the artists do use the same color profile on in game and cg models. 

The best part about this game is the new animations and expression the characters have. I imagine they can do this now cause they're not developing simultaneously with the 3ds.

I like bowser's new smash ultimate model, he looks far more menacing where as in smash 4 he was too bright,colourful and cutesy.

He looks closer to the promotional arts than before

The whole situation reminds me a lot of Splatoon 2; pre-release a lot of people were saying it looked the same as Splatoon 1, but once it came out it became clear it was a significant upgrade.

And actually I'd say Dark Samus is the bigger upgrade.

They look the same to me

Are you serious? I mean why don't you take a look at the hair between those two models it will give you a small hint of huge improvements. 

The biggest change is the lighting. That is night and day better than Smash 4. Animations are are vastly improved. Textures, shaders, and overall level of detail are also higher, but not as improved as the lighting and animations. And most models probably have a somewhat higher polygon count, but that is probably the least significant upgrade.

Of course Dark Samus would see a huge improvement. Your model is going to get a lot more attention when it goes from assist trophy to full-fledged playable character.