I know older titles are usually the topic in this community but a lot of the popular f2p games are getting pretty old now too, and if you're like me you've been curious about some of them and even pleasantly surprised now and again.
There are many examples like League of Legends, Apex Legends, Raid: Shadow Legends etc and I usually get hooked on one and play it for some months or more. I'm not really trying to get into the "business" side of things, I personally very rarely purchase anything but if you do that's absolutely fine too.
The current ones I play regularly:
PC - Enlisted - A WW2 shooter with vehicles like the Battlefield games. Same publisher and game engine as War Thunder and has been in open beta for over 2 years now. Kinda janky still but regularly updated and enjoyable.
Android - CUE cards universe everything (I forget exactly lol) - Fun deck builder but very grindy if you don't buy shit. Pretty unique concept for a deck builder what I've come across. Short games too so good for a smoke break or the bus or whatever.
Thinking about getting back into Crossfire.
That was a super fun time during high school with a good friend about 15 years ago. Maybe it's just the nostalgia but I remember some quite fun modes like Ghosts.