DRM-free exclusive here. (Also use Linux exclusively, except for when emulating ancient computers (running the OS the computer historically used), or when trying other free OSes (like the BSDs) in a VM.)
These days, I only buy from GOG, with the exception of one game I bought from itch.io, which I consider acceptable. I do not consider Steam acceptable, as the service explicitly supports DRM and even provides it, and I don't buy from Humble Bundle because they support DRM (by promoting bundles of DRM-encumbered software, and bundling DRM-encumbered software with DRM-free software).
As for itch.io, the thing I like about the store, and why I think it complements GOG nicely, is that it is not curated; this means that itch.io has a much bigger selection of games. Of course, there is a downside to that, and itch.io likely contains games that are not that good (and some that are DRM-ed, or for which only the demo is availabe there, with the full game sold exclusively on a DRM-ed platform), but then there's GOG, with its heavy curation, that does not have that downside.
(By the way, the game I bought from itch.io, Secret Little Haven, is the sort of game that is likely too niche for GOG; it's also very unlike the games I typically play.)