Getafix (Panoramix in French) is the resident druid of Asterix's village and the maker of the Magic Potion that grants the superhuman strength necessary to fight off the Romans. Only he knows the formula, as a druid can only pass on the secret to another druid that he deems worthy. As such, he is an invaluable asset in the fight against Rome.
A good voice for Getafix should convey both wisdom and eccentricity (he is not without them), and for me, those who convey that best are the unknown, Geoffrey Russell, Robert Watson Barr, Peter Hawkins, George Birt, and John Innes. However, I'm going to give my vote to Russell, as, in addition to those traits, he gives a great air of authority to his voice without it going over the top or sounding out of place.
As for the others, Danny Mann overdoes the eccentric side (even before Getafix goes insane in the actual film); Geoffrey Bayldon has good delivery, but makes him sound a little bit too grumpy; Allan Wenger is decent, but is a bit too laid back; Jeff Bennett makes him sound somewhat pompous and artificial (and isn't helped by having the least material out of anyone); and Paul Bandey's Welsh version is fun to listen to, but tends to play up the eccentricity more than the wisdom.