Way ahead of its time for when it released. If you read into the development history, Microsoft had a team that was so insanely passionate and dedicated to what they were making that it far surpassed fighting game stans and it was shat on upon launch which turned a lot of people off. It was secretly a great game, and over time and updates, it grew increasingly insane and powerful.

It's so powerful, it barely runs on the Xbox One's it was distributed on originally. It's a title that was crafted for Project Scorpio. The only thing bogging this game down was the dependence on DLC to continue content.

Potentially the most well thought out and accessible fighting game I've ever played with one of the most bat shit insane and varied cast of characters I've seen. Seriously, think of a gripe you have with a lot of fighting game mechanics, chances are Killer Instinct has a very intuitive solution for it.
Things people still beg fighting game developers to implement is stuff this game has been rocking since 2013 and is still ahead of the curve with its net code and some genuinely revolutionary things that it has such as the Shadow Labs (Extremely deep AI learning tool to fight data versions of other players offline that's scarily accurate and organic).

Many people shit on the corny visuals and all that, but all of that's just superficial. Beyond that lies a game that manages to be both simple and intricate simultaneously. The combo breaker mechanic is a genius idea that keeps you on your toes for both the person in advantage, and the person eating an eternal mix up that always makes every match feel extremely different with the sheer variety of mind games and general things you can do with it.

"Should I risk everything and use predictable moves to bait out a breaker? What if I bluff and scare them into doing nothing since they'll think I'll use a counter breaker?"
"Should I go for safer moves but miss out big damage?" etc etc
The scenarios write themselves.

Some newer players write it off as no execution mashing or just being lucky if you guess right and break away from a combo, but that's a severe misunderstanding of how it works. There's 100% execution beyond a certain point that keeps the combo system tense due to some really punishing requirements, some being as tricky as just frame moves (Having only 1 frame to execute a move). As for it being luck based, it really isn't, you can be punished extremely badly if you just mash breakers without thinking, like literally losing an entire life bar levels of damage.

Now I'm gonna just gush about the game instead of defending it from the most common complaints I see.

The cast is so much fucking fun, its gimmicky bullshit galore but in a way that doesn't feel unfun to go against. There's a character that functions like a tag fighter in a 1v1 format with assists and multiple character's health bars to manage for example, shit can get nuts. Or another character that's a weird ass fucking RNG Frakenstein that can completely change gameplans and approach at any given moment and has to bash their head like a dumbass to be able to make the RNG more in their favor at the cost of damaging themselves.

I'm not the greatest player and I'm still a bright eyed puppy compared to all the people still actively playing this but it still manages to be rewarding nonetheless, even when I get the floor wiped with me. The game holds your hand pretty well and tells you very explicitly what you did wrong, such as timing things wrong, etc. You always feel like you learn something new in a match.

Even outside the fun as hell combo system, the normal moment to moment gameplay is fucking awesome and you never feel like you've quite won yet with how easy it is to lose all momentum you have going for yourself, making every match feel like a genuine battle of wits with how intelligently thought out every mechanic is. You can never really steam roll anyone in this game consistently even when you have an entire life bar on them with how easy it is to eat shit off your short comings.

The music too is fucking great, it's some of the hypest and most visceral shit to me with how it dynamically changes off the intensity and actions of what's going on and even has a weird rhythm game-esque element to it at times. It honestly makes me excited every time when absolute banger parts of songs start playing when shit gets really clutch or neck and neck.

To go on another tangent, I absolutely dig the stupid bombastic presentation this game has. It feels like a parody of fighting games with how melodramatic and in your face it is. The characters are a weird mish mash of a child's monster action figures with how shamelessly tropey they are. Not for everyone, and people who prioritize aesthetics in a fighting game rightfully wrote this off, but I love it, this is seriously one of the funniest games to me.
You know how sitcoms or shows in general tend to have fake in universe video games that are really stupid and juvenile? That's basically what Killer Instinct is in that regard.

Yeah, you can probably tell I like Killer Instinct a lot. It's quickly become one of my favorite fighting games and might potentially be my favorite in general the more I let it sit, at least for 2D ones.