Android. I have an ipad and I hate how restricted you are in iOS. There’s no (official) sideloading of apps (yet).
Browsers suck hard. Everything is basically Safari with a skin. I wish I could get Firefox with full extension support.
I feel like I’m constantly battling the OS when I want to do something that seems really simple.
Case in point: you cannot remove photos from your photostream (or whatever it’s called).
You can add them to albums, sure, but they will also stay in the main feed. I want to move them and unclutter my feed.
And finally, and this might be an unpopular opinion, but I don’t think iOS is intuitive at all. I constantly have to google how to do things, only to often find I simply can’t, or not easily at least.