Ailerons, or the elevator?
Rudder …
The lateral component of the lift force (only?*)
You can turn with ailerons not using any rudder (look at a B2, for instance). You could also do a turn at 90 degrees AOB without any user / fin being required.
If you just use ruller, you’ll be mostly relying on the dihedral to make the ailerons work for you (secondary effect of rudder).
Most of people learning on a PA28 only use the pedals for ground (taxi / takeoff / landing) and sideslip.
I think there’s a bit of “rudder” (as in a boat or in an airship) effect, but that is fairly small. The B2 obviously doesn’t need it, for instance.
Also let’s say you are in a turn, and put the rudder completely straight. What happens?
According to Wolfgang Langwiesche it is the elevator (B2 has one), the rudder only maintains symmetry and elevator only banks: I mean why you bank left you just translate left while banking, as in you do in sailing boats, why on earth do you turn?
For sure it is not the rudder and it is difficult to test ailerons only for aircrafts without elevator…but it is easy to see how elevator force helps the rate of turn
From an engineer perspective, it is all coupled into shape and relative speed that the choice of 3 controls axis is just arbitrary as horizontal vs vertical
None of the control surfaces make the plane turn directly, however in combination the controls allow you to position the wing for level, turning flight.
When I was a young boy, I had at least two model aircraft with rudder and elevator only. They could turn, they could roll. 
The first radio controlled aircraft had only rudder, which is essentially all you need to control an aircraft ( not anywhere close to optimal, but still).
But, look at the birds, no rudder.
Newer fighter jets have thrust vectoring. More like outboard marine engines. Much better turning ability.
You turn with whatever controls you have available.
Silvaire wrote:
None of the control surfaces make the plane turn directly, however in combination the controls allow you to position the wing for level, turning flight.
That’s a nice way to put it
The ailerons obviously. It can’t be the rudder because I had entire flying lessons without using it once airborne. The elevator is mainly needed for a level turn and avoiding to stall while turning.
Obviously Silvaire had the most elegant answer above.
Of course, a turn is initiated by a resulting air force acting sideways onto the aircraft. It’s completely irrelevant how this was initiated (rudder, aileron, spoilers, cog displacement, asymmetric thrust, asymmetric drag, asymmetric lift, wing warping, shitty rigging… ).
