The DKC games are amazing and in a lot of ways better than the modern NSMBU series in terms of graphics, level design, difficulty, and music. However, the simplicity and the sort of magical vibes that 2D Mario games give make it a better series for me. There's also more variety in terms of powerups in 2D Mario. So I choose 2D Mario

I personally think that as a series, Donkey Kong Country is superior.

I always found DKC's controls/physics, variety, music, and atmosphere better than Mario's, and while all 5 main DKC games are great, I'd only consider Mario Bros 1, 3, and World to be on the same level. The NSMB games are merely decent to mediocre, none of them are in the same league as any DKC game.

Well, DKC1, DKC2 and DKC3 are amazing games.

But Mario has more 2D games, and I like a lot of them. The NES ones, SMW, NSMB Wii, NSMBU... So I'll choose Mario here.

Mario to me.
Tried Tropical Freeze but get bored, while NSMBU was pretty god. Maybe I'll do another play soon.
Still, SMB3 is my favorite one, followed by SMW.