I'd say a winger generally covers any player in a wide attacking position. You have different types, but I feel there is a bit of pickiness for the sake of it here that shows nothing more than who the FM players are
One player I don't think is a winger though is David Silva. He used to play a wide position for Valencia in his early days but moved to a central position and during his time at city he has predominantly played centrally behind the striker as a playmaker. IMO Mata is not a winger because he also began to play centrally for Valencia and has clearly preferred playing centrally for Chelsea by way of always drifting in.
I've gone off topic there completely! I think Nani is probably the best because his best performances are better than the other players best. Valencia is definitely consistent though and offers a different option. A mention to bale also who plays in a weaker team and still supplies loads of chances and has the potential to be better than Nani IMO