Probably half the best games I've ever played have been free, to be honest. Roguelikes do a lot of heavy lifting there. Stuff like CDDA, DoomRL and Nethack are absolutely crushing it compared to all those half assed cashgrab roguelikes with little upside beyond a shitty UI and tileset for normies.
PoE, as much as I hate what it's become, is still probably the best diablo clone on the market by virtue of content variety and was even better for quite a few years.
Warframe falls into the same category for me when it comes to shooters. Forget about these games being multiplayer, just go into them singleplayer, avoid guides and have fun killing shit and being surprised. You can easily get dozens if not hundreds of hours out of them that way. Certainly more than I ever got out of something like Halo.
Flash gets a bad rap for all the shitty low effort stuff on there (and it's defunct now anyways) but there were and still are quite a few excellent games made there by people investing a lot more time, talent, and skill than more than half of the garbage people pay for these days. People especially shit on the 'idle' games but a lot of those are actually excellent management sims. You can't look at something like kittensgame.com and tell me it's a dumb baby game while praising any of the 'streamlined' AAA games that have come out in the last 20 years.
To be fair I've gotten a ton out of abandonware or romhacks over they years as well, but even without that I think you'd be in more than good shape as long as you're not looking to play 100 hours a week for the rest of your life. And realistically if you throw in a budget of say, 10$ a month for really cheap or heavily discounted games, and you're golden no matter how much free time you have or which genres you want to stick to.
Edit: I ought to plug
Transcendence here too. There's plenty more stuff like that out there if you go looking, but I've forgotten most of it over the years.
