I second the Thief games, especially the first two. The graphics are a bit old but the stealth aspect still holds up very well.
Splinter Cell series - I played only a couple of them but I enjoyed them and they are all about stealth. I liked the Chaos Theory (apparently pretty hardcore compared to others especially later ones) and Conviction (you can play the whole campaing in coop with your friend, which is awesome - one is an American the other a Russian spy, the downside is the campaign isn't very long - but still very satisfying especially when playing together) the most.
I know you can play SC:Blacklist campaign with your friend as well but my SC-specialist mate said it's not as good as Conviction.
From the more recent games, the one that for me got the stealth aspect spot on, and gave me a lot of joy, was Sniper Elite 4. Moving (and shooting) silently in this game is as essential gameplay-wise as it is in Thief games, especially on a higher difficulty. I'd say it's more enjoyable to 'roleplay' and take the enemies down silently and most of all, it greatly prolongs your in game lifespan (I think you get mission achievements for that too) than to go all guns blazing (which can be a lot of fun too, provided you know the map or have some sort of plan). You can set traps and create diversions and all. The maps are huge. Not a lot of them, but their scope and design makes up for that. They are very well planned and satisfying, with lots of enemies of various kinds, including armored vehicles. And to top it all, you can can actually coop the entirety of the campaign with your friend, which practically doubles the fun. Sniper Elite 3 has pretty much the same stealth mechanics, but the maps weren't so good.
Tom Clancy's Ghost Recon Wildlands: not a stealth game per se, but its stealth aspect is pretty essential to the success of many story missions and if the mission objective is to remain undetected and you'll raise the alarm, then mission failed. In this game when you make too much noise, someone will take notice, there's an everpresent faction called Unidad (a kind of special forces militia that control the whole country) which will always find a way to kill you, so it's advised to remain prone and silent when near them.
GR:Wildlands is one of those rare examples of a game that you can play multi coop every mission on the world map, the whole story campaign and side missions as well, with a group of friends. I don't know what the upper limit is but we had a group of four and it was EPIC.
There are also the old Commandos games* and their western-themed clones Desperados were good, if you don't mind the dated graphics. Rather difficult and time consuming though. Making mistake and alarming the enemy near the end of a mission results in starting it over. I think of all the stealth games I ever played Commandos games were the most difficult / frustrating, but also hugely satisfying. There was also a Robin Hood game based on the same/similar stealth engine, seemed decent but I didn't play a lot of it.
Desperado 3 is the most recent but I haven't played it, might be worth a look.
*I think they require some modern fixes / get-arounds to run on the modern machines properly
blivion - MGSV:TPP is fun to sneak around in, but I dunno. I guess I prefer games with a stealth option instead of a game surrounded only around stealth. Like I said, I think I like F:NV's stealth the best.
