AndroidOS.
Why: well it's built on Linux kernel so first good thing.
Due to open source everyone can get cheap (under $50) smartphones running on Android with millions of app to access. We can create custom ROMs.
Your one app can target billions of people around the world and so much more...

PS: half of the Play Store apps are shit.

Have you ever tried an android phone for 50 euros?
Have you ever compared that to a windows phone of 50 euros?

Android is so heavy that most of the <$100 dont support android.

Good points. Although not entirety true.
The core and clean aosp is quite usable (for what it has to offer). Far from ideal, still fragmented, google, after all, is behind it, but better than what you get when buying prepackaged device. It is Google's set of apps and OEM's crap that dramatically cripple devices.
E.g., installing custom, source built, OS without gapps renders a usable (performance-wise) device. I can easily switch between apps without them being unloaded (open a few browser tabs and still return to my eBook in the background without it needing to reload (it is an older, 1GB ram device))
Playing with this for a while yields a simple conclusion: gapps alone cripple performance, regardless of underlying OS.

There are disadvantages, some apps desperately depend on google services. If something displays map data it usually fails. There are possible workarounds (microG) but I have no time to look into it and test.
Moral of the story: gapps suck.

@Codex404 atleast you can own a smartphone and can use IM, YT, social apps that's what most people care about

@nnee agree with you

@const and what of those are you missing? I think only snapchat is not available on winrows 10.

@MrWouter I like android go. It does its job.