The 3 that are in the drawing together... the 2 on the right are for mode (where air comes out and for air mix (sometimes referred to as "blend") are for temperature for the passenger's side, the other one is also a air mix for the driver's side.

The one by itself in the drawing is for recirculation.

Any the first 3 would be easier to replace when the dash is removed while replacing the heater core, the driver's side is the easiest one to replace if not removing the dash, the recirculation one would a little easier but not by much...BUT I would also highly recommend replacing the AC evaporator that's in the same housing with the heater core, if the evaporator ever starts leaking in the future you will be doing the same work all over again to replace it, which is a big job and the cost of a new evaporator isn't all that expensive.