The note is easy but you have to know your tenses and verbs inside and out.
The diary all-round is usually not too bad (probably the easiest one).
Formal letter can be tricky as well as dialogue. So probably the ones that people do the best in are the diary or note
The note is the easiest as you're not expected to expand on the points, whereas in the diary entry you are. DC is harder than diary and note, and letter I think is the hardest. That said, it might be easier to pick up marks doing DC or letter because I doubt as many people will attempt them. I would recommend doing two anyway because you'll have the time.