How tough a level is will depend on the player, what may be a relatively simple level for one person may be a nightmare for another player to complete with 3 stars. Listed below in order are levels most players will agree were particularly tricky and challenging to complete with 3 stars.
Level 1-21
Level 2-12
Level 3-5
Level 3-6
Level 3-19
Level 4-14
Level 4-19
Level 4-21
Level 5-20
Level 6-14
Level 7-11
Level 8-10