Steam because of the size of their library and their support of open source stuff. You would also be able to easily integrate with a steam deck if you ever get one.

GOG because of how you can truly own the games that you buy.

Steam?

Humble bundle, idk how much free time you have, but 2 games a week at just under 2 dollars per game, you won’t easily run out of new games to try.

Not all of them are winners, maybe you get less than an hour out of most of them, but when one hits it makes the whole deal worth it.

Activate via the steam interface and keep an eye out for their sales for games you truly need to possess.

Steam is very good at selling you stuff you may never get around to playing, humble bundle does that but much more affordably