I’ve got a couple in particular that I really like to use, depending on the student:
Nelson’s Blood (traditional) is great because it’s a simple melody, minor but can be quite lively as it’s a sea shanty. There are also some easy harmonies that lead straight off the main melody so it’s a good one for easing people out of their comfort zone.
Son of a Gun – remembered this beauty a few months ago, an old favourite that Nirvana made famous. Two very simple melodies but a key change between verse and chorus plus room for some nice, slightly harder harmonies focusing on the 3rd above.
Thanks for the other suggestions, made a note!