I first wanted to start writing by saying "One of the best..", but I realized this is "The best..."
Ultrakill is a game that if you are thinking of getting, or just reading reviews, just buy it.
This is the best FPS I have ever seen, It's like DOOM, but with 3 polygons for graphics. This is the only minor problem the game has, ugly graphics. Now the good parts:

A fast-paced, bloody, adrenaline-inducing, male hormone generator. This game has extremely good shooting mechanics, gun mechanics, movement mechanics, and controls. You can, and will jump higher than the empire state building accidentally, at least once. You will see nothing but blood on your screen. You will kill everything that moves. You will solve (optional and easy) puzzles. You will enjoy its music.

For the "Like DOOM" part; Yes, just like that, but way faster, way more bloody, way more satisfying, and way more ultrakill. If you think that the graphics are bad, think again! The game is so violent that you just can't notice, Your focus will be on your 11-ish guns, 2 bars, 2 melees, and the abundance of movement abilities. And the interactions between all of them.

Watching someone play is like watching people play chess, you will not understand what is happening, and the only good YouTuber that captures the game's feeling is Max0r.

You have a style meter, just like in DMC. This one has a level after the triple S-es even. The meter has multiplication bonuses for how frequently are you using the same gun up to 3x, and another multiplier for your current movement speed up to another 3x.
I will just say two of the most basic but bad-ass things you can do in this game. First, you can throw up coins, and shoot them, multiplying the damage and ricocheting it to a weak point. Second, you can parry bullets, this will send them back to the sender even faster and explode on contact. You can even parry your own bullets to make them deal more AOE damage.

While playing this experience you will come across a Crash Bandicoot level in the middle of a bloodshed. (Not even a spoiler, there are much more easter eggs in the game, probably more than any other game)
If you are still reading, go to the steam page and buy it now.

PS: After this life-altering journey (game), DOOM is played with Minecraft's music.

I am more than pleased with the Open World/Story, but some features are just the worst.

Who thought it was a good idea to make the "interact" button half your fucking keyboard.

The optional guns are put back into storage at seemingly random, making me check if my guns are still on me, or getting caught in situations where I need my weapons but I don't have them.

Some combat situations are just plain unsurvivable, When you're caught In a crossfire you're toast.

The Wanted system is absolute garbage, you get the same punishment for walking into someone with your horse and somewhat offending a citizen to shooting down the entire street.

There isn't much you can upgrade or buy in that manner. But you can go hunting for perfect snake skins with special arrows that you need to craft 5 at a time, with feathers you get from birds, for longer than the story, for +20% to your horse's core HP drain, which is negligible.