Number one for me is definitely Celeste, both in its subgenre of twitch platformers and 2D platformers in general. I would guess that VVVVVV sits in that subgenre too as another favourite.
Mario has to be up there, and while the original SMB has a near-perfect opening and deserves buckets of praise for still somehow being so very playable, I have to put SMB3 over it to take the classic platforming crown. I am also though a big fan of Chip 'n Dale: Rescue Rangers for NES - especially in co-op.
With endless runners I was only half joking in the Switch thread about Robot Unicorn Attack, but my real favourite is Canabalt - and I genuinely think it's brilliant.
I'm a big fan of the Klonoa series - Empire of Dreams on GBA gets the top puzzle-platformer position, while I'm torn between Door to Phantomile (either PS1 original or Wii remake) and Lunatea's Veil for 2.5D platforming (which is really much closer to 2D than it is 3D).
To further stretch the boundaries of 2D platformer, Mario Maker 2 and Little Big Planet 2 are the best of the ostensibly 2D platformers but really robust creation tools (and while LBP2 mechanically isn't a very good platformer, the physics work very well for the physics sandbox the game actually is).
I think it's probably then a stretch too far to consider Metroidvanias, even if they can heavily feature platforming!
Gemini73 wrote:Rolling Thunder
I absolutely love Rolling Thunder, but I'd never call it a platformer!
However thinking of the arcade I do wonder if I should include Mappy - because that game is boss.