Spotify for almost everything. The only other time I use a different app is Pandora and that's for comedy radio stations (Spotify and iTunes Radio both throw in music into their comedy stations and I just want stand-up, not funny music).
I will sometimes use iTunes Radio while driving but 99% of the time I'm using Spotify.
Pandora mostly. Haven't really looked at others as it does what I want from it, which is random music from a genre/style for background and I don't particularly care about looking for a specific album or artist.
Can't speak to bandwidth as I typically stream only at home or in the office, both of which have wifi.
I was using Amazon since I already had Prime, but the lack of selection made me try Beats since I got the 3 month trial with AT&T. I really like Beats, the app is really nice and of course has every artist you can imagine. Don't know about data usage, I download everything.
I am using Beats music right now because of the AT&T promotion. I decided to switch my primary music source because of my issues with iOS 8 and iTunes 11.4 but that's a different story for a different day lol.