Mgiese84 I think it is never objective to tell. I have painted around 150 episodes and can only share my experience from those I painted yet. S19E7 (Covered bridge oval) I think was the yet most difficult I tried. I also found S3E9 (The old Mill) and S20E2 (New Day's Dawn) quite a challenge.
On the other hand, Bob mentions that the painting will give you quite a challenge at the end of several episodes which I didn't try yet. If you are a good in drawing, then the paintings with buildings might be more easy for you. I have biggest problems in paintings where the knife is used a lot. So maybe to me paintings with mountains might be more challenging than paintings with buildings.