I see DKC2 being mentioned, so I'm just gonna throw this out there.
It is a masterpiece and if I taught a course on Game Design it'd be one the chief examples I'd use. It isn't just "fun", like DKC1 and DKC3 is. It is meticulously designed. There are no leap of faith jumps, which are littered throughout the other two titles. Most levels are designed to be ran through at full speed in ways Mario or other great platformers aren't. In Mario, you might need to jump on a flying enemy to make it over a gap, but you'll have to stop and wait for the enemy to reach the proper height for you bounce off them. In DKC2, if you just book it through the stage at full run speed, that flying enemy you need to bounce on will be in the exact position needed to make the jump. Watch a speedrun, it is magic. And yet the game never feels easy, because it also wants you to stop and explore everything.
In DKC1, there are many secret areas found in pits that give you no indication they are safe to fall in; there's little rhyme or reason to the secrets in DKC1, so the only way you could ever find them all without a guide is by jumping into every single pit in the game. Compare this to DKC2, which always puts its secrets in "plain sight." But that doesn't make them easy to find. No, the game revels in hiding a giant sparkling gold coin twice as large as your character in areas you feel stupid for missing. DKC as a series obviously owes a lot to the Mario platformers, but the huge, sparkling coins found in the NSMB series are a clear example of the inspiration flowing the opposite direction. Making your secret collectible in every level absolutely gigantic was a master stroke. When the player misses it, they feel compelled to go back in and find it, because how can something so big and shiny be so easy to overlook?
Then there are the level themes. There's an amount of creativity here missing from most other "great" platformers. Even Mario Bros will tick off all the cliches: a desert, a forest, a snowy plain. DKC2, instead, sticks you in a swamp with a half sunken pirate ship, a giant honey-filled wasp nest, a rusty, rickety amusement park and, of course, a floating bramble cluster in the sky. Even the 'cliche' areas, such as the dangerous lava filled volcano land, are turned on their head. What is usually reserved for World 8 becomes World 2! Lava is the least of the Kongs' problems. I can't even begin to tell you how disappointed I was when Donkey Kong Country Returns made the volcano land the last world, like every other platformer.
