Steam, best discounts that no one else can match, reliable downloads, backups (for game data), cloud save etc.
I have one game (Battlefield 4) on origin. Origin has on-the-house deal sometimes, which give premium games for free.
It also has discount sales which can match with Steam sales. (But it doesn't show the time remaining until the sale is over)
Origin doesn't have game data backup, but it recognizes copied game files.
It has cloud save for games that support it.
https://en.wikipedia.org/wiki/List_of_Origin's_On_the_House_games
Origin games are not available on Steam.
Ubisoft. I have The Crew on Ubisoft. Plus as a part of their 30th anniversary, they were giving away premium games for free. They still have 4 titles left.
Haven't seen much discounts on Ubisoft, but Ubisoft games on steam get big discounts every now and then.
https://club.ubisoft.com/en-GB/ubi30
Ubisoft launcher is good and reliable as well.
Ubisoft games are available on Steam
GoG, I haven't used. But GoG titles are also available on Steam.
In origin, there is a good chance of downloads getting reset if pc got shut down or if the bandwidth gets throttled.
For me, I don't have a high speed broadband, power failures happen often in my place.
If something happens to my hard disk, I don't want to be downloading all the games from scratch.
So I rely heavily on the backup feature of steam.
Bottom line is, I trust only steam. It is solid. All purchases in one place.
I have one game on Windows store which is Forza Apex. The downloading was smooth, even though it got interrupted.
But it is 16 or 18GB of data with no option to backup and restore.
It's a shame that Windows store doesn't support this option. All the other launchers recognizes copied game files.
I will never make a purchase on Windows store unless they implement some feature to backup and restore game files.
If you have a tonne of bandwidth, go ahead and try out games on different platforms.
If you want the best deals in games, reliability and what all I mentioned previously, stick to steam.
For me, I am going to keep it in steam, all in one place. I would consider another platform only if it is not available on Steam like I did for Battlefield 4.