Ready or Not and Ground Branch are the obvious choices.

The Stalker: Anomaly mod pack is also quite hard, less so because of tactics and more so because of bullshit. But its free.

Wildcard: Ghost Recon: Wildlands. On default difficulty its a stupid popcorn shooter, but if you crank all the difficulty settings up (they call it „advanced mode“ IIRC) it becomes really fun. Most people like to shit on the typical Ubisoft formula in Breakpoint and Wildlands, but I enjoyed my time with both.

If you are not married to 1st person, Zero Sievert and Doorkickers are worth a look too.

And maybe also something like ARMA3?

SWAT series is an obvious pick. I haven’t played Ready Or Not but everybody says it’s basically a modern SWAT series game.

Wildcard picks that aren’t FPS but activate the same brain neurons:

Doorkickers: A top down game to plan out clearing structures with different scenarios. Think the planning stage in old Rainbow 6 games but that’s the whole game. Technically you can give real time orders but the fun comes in trying to clear a whole level just with your pre-planned maneuvers.

Full Spectrum Warrior: The player controls two army fireteams, switching between them for an over-the-shoulder view, and gives commands in real time. The player doesn’t directly shoot. Starts out with basic tactics like one team suppressing and the other flanking, and then gets increasingly complex. A character getting hit and wounded means carrying them all the way back to a medical station. A character dying is a mission fail. Can get tough.

I agree with what many others are saying:

• Ground Branch gameplay is very reminiscent of old Ghost Recon. Probably the standout feature is the customization/loadouts.
• Ready Or Not is like SWAT, which has a lot of similarities to old Rainbow Six. The atmosphere is a lot more grim compared to the others. This also has decent AI teammates and commanding over them when playing single player.
• Arma 3. In a lot of ways, the Arma series can be compared to Ghost Recon, but with a ton of extra realism/depth/scale/gameplay possibilities. There is also a ton of single player content across the base game, DLCs, and community-made missions and mods. Or you can make your own missions. The official single player content has a surprising amount of lore, if you are into that. Compared to the rest of my suggestions, this is the only game that isn't in early access (and it shows). There is so much more to say about Arma 3, if you have interest, but I don't want to write too much here if you don't need to hear more.
• Zero Hour has had a lot of its thunder stolen by Ready Or Not, but it might still be worth looking into if you want more options.

Glad to see all these recommendations -- I hadn't heard of Ground Branch, I'll have to check it out. This post is reminding me of how great the original Rainbow Six and Ghost Recon games where.

Also, there is a forgotten entry from the original developer! There was a video game tie-in for The Sum of All Fears movie, and it was basically a reskin of the original Ghost Recon with indoor maps. I remember playing the demo back when it first came out. Looks like it's on Steam, and the critics agree: it's meh! But I want to play it anyway now. https://store.steampowered.com/app/19810/The_Sum_of_All_Fears/

Side note: does anyone know a legit way to get Rainbow Six: Rogue Spear? I swear it was on Steam or GOG at one point, but I can't find it now. Thankfully, I have it on a CD from when it first came out, but I wish I could recommend it to other people.