I haven't tried GOG 2.0 beta, yet, but currently the original Galaxy is my favourite, because it's optional. I use it for GOG games that have achievements, but for older titles i just use the old-fashioned desktop shortcuts.
Steam has the best storefront/recommendation/discoverability systems. I would say I'd like Galaxy to get something similar, but really, GOG catalogue is still significantly limited compared to Steam, so it doesn't need those types of search functions (yet). The curation aspect keeps its library pretty limited- If i look under "strategy" or RPG games, i usually get under 15 pages of results- and that's a small enough number that I can actually browse all of them if I'm really curious.
Origin needs a better search function on its store and to stop shoving subscription options in my face.
Battle.net... i use it to play StarCraft II (occasionally, when I feel like playing the campaign) and might give it another whirl for Reforged. It's a 2-game launcher that's limited and self-contained, and I don't really care about it.
-I'm yet to try the WinStore and Uplay, so no comments about those.