No love for
Outland? Of all the 2D action games I've played in the last decade, it's one of the best for the sheer joy of moving and controlling the character. The game has great bosses and a nifty polarity-switching gimmick, and it went tragically underappreciated on its initial release, in part because it had the misfortune of coming out right before the big PSN hack and then not seeing a PC port for 3+ years.
Beyond that, I'd mostly reiterate other suggestions. Volgarr is tough but an expertly crafted take on a certain type of tough, arcade-esque 16-bit action. Castle in the Darkness is a phenomenal (and also sadly underappreciated) riff on Castlevania, with tons of fantastic bosses, excellent level design, and a killer 8-bit soundtrack. Shovel Knight is a pitch-perfect take on the Mega Man formula, also with a killer 8-bit soundtrack.
I mentally file them away as other genres, but Mark of the Ninja (action-stealth) and Ori and the Blind Forest (Metroidvania) are also both excellent.