I flew two SFO-LHR round trips on Virgin, one CDG-SFO round trip on UA, and one LHR-ORD round trip on AA in 2001-2002.

Virgin had the best entertainment system. I was very amused with various game.

With regard to everything else, especially service, I would take UA above VS and AA. In particular, on the SFO-LHR legs, the VS flights attendants were absolutely invisible in coach between dinner and breakfast (not even offering water), and in general, while polite, the FAs were not particularly friendly. Even though I was simply a college student at the time, I was treated much better by the UA flight attendants.

Also, I think the 777 aircraft on UA is much nicer, especially if you can get either the window or the aisle seats, than the 747 on VS. Lastly, I can't say the quality of food is particularly better on VS; it didn't seem very different from UA.

So I would rate UA first, VS a somewhat-distant second, followed by AA for coach transatlantic travel -- at least in my experience!