Depends. If one character is lacking in physical attacks you should probably give them the attack magic. It doesn't really matter who you give the materia to, sometimes handing out the materia may just be a matter of the number of slots you have and the usefulness of them. The characters may have greater MP than others so you might want to give them the materia.

No certain materia is placed for any particular character so it may not matter who you give them too. It mostly matters on the usefulness the materia will have in battle and how they will help your characters.