2D Mario because as good as DK games are classic 2D Mario (SMB, SMB3, SMW, SMW2:YI) beats it out, TF is the best 2D platformer overall though.

Well... sort of... 

There are pre-made levels, but to a large extend, the point of them is to show you neat things you can do with various tools.  They're designed as much as tutorials as actual levels.  

While I haven't thought much about it I guess I actually do prefer DKC. Only played the original trilogy, but I pretty much liked those games more than any mainline 2D Mario.

3D Mario > DKC (2D) > 2D Mario games.

Also if you look at resent 2D mario games, vs resent DKC games, its not even close.
tropical freeze is such a fantastic platformer, that no 2D mario game even comes close.

I'd take Super Mario over DKC. And even then, I'm mostly taking the older games into account here - anything post SNES doesn't really stand up against the classics (for either series). Both fantastic, but I've always preferred Mario's level design and general presentation.

I think Donkey Kong in general is one of Nintendo's finest franchise, but in this contest Super Mario wins. It's a question of numbers. We're talking five games versus, what, 13? And two of those 13 -- Super Mario Bros. 3 and Super Mario World -- are two of the best games ever made.

Donkey Kong Country 2 is damn hard to beat.

Charming art, solid gameplay mechanics, lots of novelties and great thematics, game pacing and more leads to a damn solid package.

2D Mario really only has Mario 3 and Super Mario World (and Yoshi;s ISland if we count that.)

DKC has the original SNES trilogy AND the Returns/Tropical freeze combo.

I like the original DKC trilogy more than SMB3, SMW, and Yoshi;s Island.

I like Returns/tropical Freeze more than the NSMB (Even with the latest one on Switch/WiiU, which was outstandingly good)

DK wins the classic and modern rounds to me. DK all the way, even if Mario is still amazing.

In case of gameplay I'd say Mario, in case of atmosphere (music, level design) I'd say DKC. Tropical Freeze beats NSMBU in almost every aspect. Also DKC2 has one of the best soundtracks ever. 2D Mario surely had a bigger impact on the industry. Difficult. I voted DK.

The original DKC was fantastic when it launched; nothing groundbreaking aside from the wonderful visuals (and stellar soundtrack) but good, solid platforming gameplay and the debut of Diddy Kong, with its only big weakness being that it was a bit short. Unfortunately, its sequels failed to live up to what the original had accomplished (my opinion), feeling way too derivative of what the first had already done, and not nearly as fun or memorable despite any minor advancements made. Having said that, they're still ok games that play well.

Unfortunately the same cannot be said for DKCR for Wii, which, stupidly, featured the addition of mandatory motion for the roll move... in a 2D platformer that relies on precision. Factor in that the game was nothing to write home about in the first place (not to mention that ridiculous "blow" move) and it left me disappointed with Retro's freshman take on the series. However, the glorious Tropical Freeze came along and eliminated the deplorable motion move while simultaneously delivering a game that was so imaginative and polished that it finally met (even exceeded) the magic of the first game. The only thing that still bugs me is Donkey Kong's slippery control, which I realize is an intentional design choice, but not one I'm fond of.

Regarding 2D Mario, the "New" games were nothing special and I didn't like SML on Gameboy, but there are very few games ever designed that have the level of creativity and excellence that SMB, SMB3 and SMW achieve. Tight controls, great level design and secrets abound, 2D Mario is just plain fun and without equal as a series among platformers.

To sum it up, even the best of the DKC franchise can't compete with SMB3 or SMW for me.