What is the best traditional 2D platformer.  

Donkey Kong Kountry, Rayman, or Super Mario Bros?  

Or possibly some other game?

For me it is close with DKC and Rayman.  DKC has more charm and more appeal to me (That and Rare's DKC was brilliant) but Rayman is very challenging and offers a great core game where as New Super Mario Bros has really went the easy route and just good for co-op in my opinion beating the wiiu version with 90+ lives

Donkey Kong Country for me. Returns is one of the best platformers for the Wii, and the originals are the best SNES games imo.

I don't really care that much for 2D Mario games and Rayman, while good, doesn't come close to DKC.

You mean the classic games or the recent installments?

Doesnt matter.  As long as they are the traditional 2D platformer style. 

The only game I have played from the Rayman series is Origins. 3/10 game.

As much as I love the brilliance of SMB3, DKCR is my favorite 2D platformer of all time, so I'll give this one to DK series.

rayman isnt an exclusive so it wont win even though its the best one (judging by metacritic)

So we could choose Super Mario Bros to be lets say the SMB3 or World? If so its a no brainer because both SMB3 and World blow the other 2 series out the water IMO.

To ME New Super Mario Bros and Super Mario Bros are two different games (series) so if you consider them the same i can't vote.

dont forget sonic and megaman

rayman is too new to compare to years of dk and mario. i wouldnt choose. i like them all.