Donovan's main problem has always been his unwillingness to run at players. Part of it is I guess is that he's not an exceptional dribbler. Pienaar for example has much better touch and close control, dribbling, etc. But Donovan's very technical in a lot of ways, he puts in a good ball (one of the mysteries of his stay at Everton is why he didn't take more corners, even though he didn't take that many I can think of at least two goals off his corners) and is a good finisher, outside of Tottenham.
Arteta is the most technical guy on the team. Pienaar's probably more technical because of his control but there are things Donovan does better. Osman is iffy, again he has better control and touch but Donovan is better in other areas. I think Donovan's better than Billy but I don't rate him like some people on here do.
Edit: I think Billy's good, I just would like to see more consistency and I don't think he's really at his best out wide.