Donkey Kong Country trilogy from SNES are timeless classics. But I must say Super Mario Bros. 2 was one hell of a great game! But I might say DK wins this round because the serie is still fresh nowadays and comes close to be as good as the Rare ones!

DKC3 was a really different game but that doesn't change the fact that it was excellent.

It's a step down from DKC2, (which is still one of the best games of all time) but it's still a great game, better than,  say, the New Super Mario Bros series, the Sonic games, or any Rayman except maybe Origins.

I saw you said no Mega Man.  What's not traditional about Mega Man, Castlevania, Ninja Gaiden, etc/  The games are just as much of platform titles as the others, just a different way to kill enemies.  Mario can stomp on enemies, but he eventually will get the fire flower, raccoon, hammer bros, or tanooki suit.  It's unfair to count out games just because you don't stomp on an enemies head to kill them.

In my opinion, it's Castlevania series, Ninja Gaiden 2, classic Super Mario Bros series, Prinny series, Mega Man series, Rayman, Little Big Planet, Donkey Kong, Adventure Island, then way way way back there after a bunch of other games not in this list but in front of some others, Sonic.   

If we look at recent titles alone, I would say Rayman Orgins> Donkey Kong Country Returns > New super mario bros (wii/2/U= all the same)

Mario > Rayman > Donkey Kong > Sonic

lol whotf voted rayman... HAHA

Depends on what you include as Super Mario Bros, the original trilogy? Do you include the new games? Mario World? Other Mario Platformers?

I am just going to go with the original trilogies of each series, with Mario 2 swapped for world.

1. Super Mario Bros 3 - made the biggest impact of any game on NES and of any 2D Platformer ever. It's also one of the best 2D Platformers ever, and even today stands up with its 8-bit stylized art work. It really shaped the way for Platformers of the future including Kirby, Mario, and DKC.
2. DKC - made the biggest impact of any game on SNES and second biggest of any 2D Platformer ever.
3. DKC2 - Most highly regarded 2D Platformer of the 16-bit era. Maybe even the best SNES game.
4. Sonic 2 - Made a bigger splash than Sonic 1, it was a solid Platformer and better than Mario World.
5. Sonic 1 - the first real challenger to Mario, and this game was really good!
6. DKC3 - while not making as big of a splash as the above titles, realistically better than DKC1 and very close in quality to DKC2. Most underrated of the series.
7. Super Mario World - good game, not really a big step up from the last, and in many ways a step down. Most got it as a pack-in.
8. Super Mario Bros 1. - It was a big game in its time, almost everyone got it as a pack-in with consoles, many played it. It really wasn't that great, I find of all the games this is the only one I get quickly bored playing today.
9. Sonic 3 - I would probably say this is better than Mario 1, but this game was ruined by bad level design despite some really interesting ideas. It also made the lowest splash of all games on the list.

Rayman? Really? That franchise is well below Kirby let alone something to stand aside the other three legendary franchises. I am not even going to bother with it. It's not worth consideration.

Super Mario Bros 3 and Super Mario World are the pinnacle of 2D platformers for me. NSMBU is the best of the NSMB series by far.

All the Donkey Kong Country games are great. They're tougher than other platformers, but never unfair. DKC and Returns are probably my favourites of the bunch.

I have a love/hate relationship with Sonic, but the highs just about outweigh the lows. The first game remains my favourite.

Rayman is just average really. The old games were nothing special or memorable, and most of the mechanics in Origins were taken from DKC games, but with added floaty physics, and floaty physics are the one thing you don't want in a 2D platformer.

Rayman 1 > All of those aside from SMB and SMB3. Easily.

So, single game: SMB, SMB3, Rayman 1, are the best.

Series, SMB is the best.