OK, so Stephen Merchant of The Office is doing Wheatley. Not a bad choice. But what it really did was make me want a depressed  Karl Pilkington sphere in there real bad.
 
But if you could pick anybody to do the voice over work for one of these companions who would you pick? Who's voice would you like to guide you through the tricks and traps of Aperture Science? And what would they be called? 
 
The thought of a Sam Jackson sphere banging on about how he's "Sick and tired of these Mutha Fucking Turrets in his Mutha Fucking Portals!" makes me giggle, even if it's not Valves style.

Jeff's French / Mexican / Italian voice.

Morgan Freeman.

mark hamill. 

The guy who was the cross-dresser in The Silence of the Lambs and the captain from Monk. I forgot his name. 

Nolan North playing a Nolan North Sphere.

Rip Torn is the answer to this question and every other question ever asked.  You may go.

A SHODAN sphere.

Patrick Stewart doing a Picard sphere!