Well, Ubuntu 1 was my favorite, but they're closing down in a couple of months.
Since then I've signed up at Dropbox (on my Ubuntu and Windows computers and the App for my Android phone) and SpiderOak (for the computers -- the Android App has mediocre reviews).
Both Dropbox and SpiderOak are well integrated with Linux (I'm using Ubuntu) and Windows. SpiderOak has the advantage of being fully encrypted, but it's slower -- probably because of the encryption.
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