Here is a link to the career leaders in eFG%, which looks at the most efficient shooters (by weighting three points more than two pointers.) If you look at the guards on the list, this may be the best way to determine who the best shooters are.
Here are some of the point guards / combo guards:
Brent Barry: .5703 eFG%
Steve Kerr: .5642 eFG%
Steve Nash: .5552 eFG%
Leandro Barbosa: .5489 eFG%
John Stockton: .5463 eFG%
Hubert Davis: .5374 eFG%
Mario Elie: .5365 eFG%
Dana Barros: .5359 eFG%
Magic Johnson: .5333 eFG% (due mostly to him being so efficient inside)
Mark Price: .5305 eFG%
John Paxson: .5277 eFG%
Maurice Cheeks: .5261 eFG%
Danny Ainge: .5193 eFG%
Again, not all of these guys were pure point guards, but almost all of them were great shooters.