One of the greatest FPS games ever conceived and executed, the sheer versatility, yet simplicity, within your arsenal is absurd, and the level design, enemy design, and generally just, the feel of the game, is near perfect. Ultrakill has such insane replay value with its main levels not to mention it has secret levels, secret bosses, alternate weapons to unlock, a ranking system, scoreboards, and even an endless mode (cybergrind is so so fun).
The overall visual and gameplay design of Ultrakill has entranced me forever, amazing game.

P.S. another thing to note, this game has an absurd amount of accessibility settings and difficulties, I don't use any myself (in fact, my cybergrind textures + visual settings might be anti-accessible) but seriously, even past the default main difficulties which are already very accessible, there are things like minor assists which include scalable auto-aim and enemy outlines/flat colors, and major assist with just so so so many options, not to mention accessible visual options with the ability to turn off explosions, change nearly EVERY SINGLE color of the hud INDIVIDUALLY, and apply color filters over everything.