Sync files across devices

In essence, I have Windows 10 desktop, laptop, iPad and Android smartphone.
What should I use to keep files synchronized? Dropbox? Similar cloud systems? I'd rather have my files on my devices and if it isn't free, at least affordable and reliable. SyncToy, BitTorrent Sync...

Thanks for any input!

What? You've answered your own question here. Try them out or google some reviews. BTSync and Dropbox are both fantastic and multi platform.
Dropbox = on the internet. BTSync = Peer To Peer.