I would say Aleksandr Hleb is by far the most underrated loss of talent I've seen as an Arsenal fan (that's only 16-17 years so maybe not saying a great deal).
The fact that he became absolutely nothing since then is just bewildering to me. Whatever personal problems and poor relations he had with Guardiola, plus the injuries he picked up after ditching us totally fucked him. He's a prime example of a player that threw it all away on his own. I suppose his main downfall in his prime was the fact he was a pretty shit human being. By the time he realised his stupidity it was already too late. Lost his quickness, his pace, his elusiveness and couldn't fit in anywhere other than Arsene's Wengerball.
As a player for us he was too technical for the average Joe to understand. All people could think about was the fact that he wouldn't take enough shots, rather than his remarkable creative, dribbling and retention abilities. I couldn't think of anyone in the second half of Wenger's tenure I would rather have playing RW for us right now if we could pluck him out of 2007/08 into the present time through a time machine.
Such a unique wide playmaker. We'll rarely see that kind of streamlined left-footed player nowadays.