There are two potential answers here, to me
Tropical Freeze is the Full Package sort of best platformer of all time. It's massive, more than most people will ever see, it flows incredibly, the soundtrack is hours long and every theme's a banger. It is as full as such a game can be without trending into the excessive.
But then there's the other answer, which is the platformer cut down to its barest, most beautiful essentials, every step meaningful and every second played one that teaches you or hones you into a more and more finely tuned instrument.
And nothing embodies that second answer, to me, like Cyber Shadow.
Cyber Shadow is what retro-styled platformers should be. Hard as nails but relentlessly fair, with a very generous checkpoint system and a death->reload loop that lasts for around a second. Every song is great. The sprite work is second to none, while keeping to NES-accurate colour palettes. The writing is good, too, and fits a lot of theme into a short script.
But it's the gameplay. Of course it's the gameplay. Every level you go into is some new thing with new obstacles for you to tackle with a constantly evolving toolkit, where the start of the game feels very precisely engineered but also relatively simple, but by the end of the first chapter you can already see the direction the game is heading, and by the end you are a flying god whose feet are carrying him through the bowels of Hell
It's such a ride, from beginning to end, and it encourages you to play again because why wouldn't you? Wouldn't you like to see if you can complete its challenges? How few deaths can you get?
Shovel Knight would be the other answer here—and I'm absolutely shocked it's not on this poll—but Shovel Knight, Tropical Freeze, they've already gotten their flowers. Everyone knows.
I'm telling you, though. Cyber Shadow? This is the real stuff