I know I'm a bit late to this but last night I finally finished Hollow Knight and the game absolutely blew me away (37 hours total and I finished at 89% out of a possible 112%). So much to do. Huge map, tons of upgrades, tons of bosses, tons of secrets, hand-drawn graphics, beautiful soundtrack and it was HARD. Huge penalty if you die twice so every enemy encounter (at least in the beginning) mattered. The controls are also perfect (jumping, dashing, etc.). The story was fantastic too - not in your face (no cutscenes) but you learned as much as you wanted to learn by exploration (think Metroid Prime).
This game is the pinnacle of 2D platforming adventure games, IMO. It's actually better than Super Metroid and Symphony of the Night. It's that good.