Radagast the Brown
No disrespect to Sylvester McCoy, who is a fine actor, but the approach Peter Jackson took with Radagast in The Hobbit films is a significant misfire for me. His soft-spoken, scatterbrained demeanor and bizarre physical appearance made the character far less believable than his Istari brethren. Radagast is certainly eccentric, but a cartoon he is not.
I would have taken a more subtle approach to Radagast's eccentricities: still a somewhat disheveled recluse, but a proud and confident wizard nonetheless. (For God's sake, no dried bird poo in his hair.) The late Pete Postlethwaite would have brought just such qualities to the character. Behind his kindly eyes, the audience would find a tender, quirky, but ultimately wise and authoritative personality.