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The obvious being the wall and corner where your sink is and again behind the stove. However I would also do that wall where the light switch is beside the stove. Go to the end of your cupboard there and then taper it down to the end of the countertop. A sort of rising step pattern.
Why? Well that area is very prone to spills and grease and so having an easy surface to clean is important. Painted areas will eventually wash off - tile wont. Second reason is that you see that bare wall when you walk in that doorway. Might as well make it look nice.
I agree with CG9196.
Your backsplash should be behind the length of your countertop. So, in your case on 3 walls. The wall behind your sink. The wall to the left of your door (but not the right side) and the wall behind your stove.
Did I pick your side???? The wife is always right!!!!!!