We'd have to disagree on the first page already, as I found New Super Mario Bros. unbelievably good.
The best 2D platformer in my opinion is Donkey Kong Country 2, mostly because it's a very balanced game between finding secrets and just finishing levels. I think the trick is to avoid hitting the extremes. DK64 is a boring hunt for secrets, and in SMB3 there's just no reason to play through most levels.
DKC2 levels treat the more concentrated players very well and never punish them for having missed a secret. There's one level at the swamp land where you can go from start to finish only jumping from one enemy to the other. You may actually keep your eyes on the screen borders the whole time to check for secret paths.
It's also the best at hiding those secrets, as explained by David Sirlin in an article I can't find anymore. Basically there's a hierarchy of secrets in every level, from lesser prizes (Banana Coins) to valuable secrets (bonus barrels) and one really huge item that somehow manages to stay hidden somewhere (DK Coin). You always know exactly what you're missing, they're never hidden in a cheap way and there are many clever hints leading to them, usually by elegant banana trails. Personally I think DKC1 sucked at this, since you had to plummet into doom in every pit if you wanted to find bonus rooms, and finding them was the only requirement.
And if you think about it, the gameplay in DKC2 is extremely simple, even more so than Super Mario World for example. Just flawless execution, really.
DKC3 is also very good at this, but level design is way too gimmicky so missing secrets and going back for them is a major pain in the ass. Flipped left and right controls? Bleh!