Games I already have:

Quantic Dream Collection
Nathan Drake Collection
TLOU Remastered
Bloodborne

What is highly recommended that I get? Please don't list games that are multiplatform since there will be a 99.9% chance that I already own them on XBO.

Uncharted 4 (masterpiece and well worth playing after the Nathan drake collection)
Crash Bandicoot
Infamous: Second Son
No man's sky (don't believe the old review scores, especially after the latest patch)
Nioh
Persona 5
Horizon zero dawn
The last guardian
Little big planet 3
Driveclub (cheap with tons of content)
Salt and sanctuary
Resogun
Until dawn

Not sure what your taste is but that should cover a little.

knack and knack 2 babeyy

@Alaric - Siosal's given a great list and there's a few I'd also recommend:

- Salt and Sanctuary, especially if you're a fan of Bloodborne. Fantastic 2D Soul-like with almost every element you'd want from a SOuls game intact, coupled with some tough but fun platforming.

- Infamous Second Son is not quite as complex and sprawling as the originals but are great fun to play. The standalone prequel First Light also great, expanding on one of the more interesting side characters and the antagonist.

- Gravity Rush Remastered and Gravity Rush 2 are quirky, very Japanese, open world games. Light-hearted and a protagonist with an unusual set of superpowers.

EDIT - FFXII: The Zodiac Age If you ever played or enjoyed it on PS2, the remastering effort is impressive.

Some of the old exclusives or less critically acclaimed games are still worth it if they're heavily discounted. Killzone: Shadowfall offers a fun and diverse sci-fi shooter campaign with some visuals that still look impressive today. The Order: 1886 is both too short yet moves too slowly but has incredible production values, great characters and gunplay. It's a great prologue story that sort of ends just as it gets interesting.

Crash Bandicoot n'Sane trilogy is worth a look..

Infamous 2nd Son and First Light. First Light is standalone DLC (related to 2nd son) and honestly the better of the 2.

also knack, cause knack 2 is going to be incredible

Uncharted Lost Legacy. Just play Uncharted 4 first.
& if you want some old school nostalgia, Wipeout Omega Collection & Crash Bandicoot N. Sane Trilogy!

I'd be careful about the amount of Uncharted (or any game) you play. Pretty easy to get series burnout and not appreciate them at all.

I'm also on the side of the fence that wouldn't recommend Knack. It's a fine game, and better with 2 people, but it's pretty basic and not exactly a must play ps4 title.