Online coop games to recommend?

ESO is fun with some friends and a guild
try Deep Rock Galactic, Payday 2, 7 Days to Die, Terraria, V Rising, Grim Dawn, Stardew Valley and maybe Viscera Cleanup Detail

You could always look at single screen / split screen MP games. With remote play together it should still be fun.
Like Vampire Survivors, or any of the Metal Slug games are good candidates for this. The person joining HAS to play with a controller however (or emulate one through joytokey or something)

If you're looking specifically for games with large playerbase, I'd suggest checking the Steam charts, as quite a few of them will have focus on online multiplayer, some of them being co-op:
https://store.steampowered.com/charts/mostplayed

Also, here you can check all "online co-op" releases ordered by relevance, meaning the top of the list will probably have a live playerbase (though not 100% of the time the game will be co-op itself, it might be pvevp or competitive in teams):
https://store.steampowered.com/search/?category1=998&category3=38&unvrsupport=401&os=win&hidef2p=1&ndl=1

I have a playlist of online co-op games I've covered on my channel, but the majority of them won't have a working public matchmaking, either for the lack of the feature, of lack of active playerbase, so I'd only recommend to buy them for co-op if you get the game together with friends:
https://www.youtube.com/playlist?list=PLXSJgzsctNy2PBsvtUzzeyX9l-QukyqTW

Personally I haven't being playing public online games regularly for a long time, last times I remember were in Nioh 1 a while ago, so I can't really recommend specific games based on that.

Last online co-op release I've tried was the just released EvilVEvil, but looks DOA regarding playerbase https://steamdb.info/app/809090/charts/ so there're actually multiple releases, but most of them won't really be able to have a working playerbase to be alive

Last public match (forced) I played was on Warhammer 40,000: Darktide, which I played on PC Game Pass. I guess getting a month on Game Pass would be a nice alternative to find many multiplayer online co-op games with a steady playerbase.

You would have to request co-op the old fashion way, but you could play online co-op with Portal 2. At the very least, the gameplay is not mind numbing.

You could give a try to Payday 2. The base game is usually cheap during the sale, there are still 20k people playing it on Steam. Buying everything is pretty expensive tho (even during sale).

Crime Boss is what Payday 3 should have been and it has several DLC for free. Besides being like Payday, it also has a roguelike mode where you build your criminal empire and taking out other gangs. You get to own property and hire people.