I think Bengleton has the best list with two possible exceptions;
Song who is Premiership standard but not quite Arsenal standard, and Black who I think came through the youth system?
To add to what G88ner says, any player signed for a minimal fee or openly as a gamble should not really be included; Bischoff, Mendez-Jiminez etc.
Diawara neither, he was bought and sold for the same amount in some sort of favour for another club and was never meant to be with us for longer then half a season; he was superb in my opinion, being the world's best ever player at the crossbar challenge
As for the very worst, my vote goes to
Squillachi who whilst on paper looked a reasonable experienced squad player, has not produced anything like what was expected of him.
Just goes to show you can never tell how a player will turn out.
The best?
A combination of transfer fee paid and transfer fee received combined with effectiveness on the pitch, I think would be
Vieira, who we sold for about £12M profit after long service.
Henry too an excellent bit of business but just edged out as we arguably made no profit, his transfer fee for the sale being £5M more than we paid for him, but he had an up front £5M bonus the year before.
In purely business terms, Anelka is an extraordinary tale; paid £0.5M sold £23M!