Subjective, but this is what I think; (take this very lightly! - I think a lot of them are equally hard/easy due to different issues - whether you're a beginner or an advanced player, fingerings, embouchure, breath control, temperamental instruments etc)
Organ, Piano, Pedal Harp, French Horn, Violin, Clarsach, Northumbrian Pipes, trumpet, Viola, Clarinet/Oboe (oboe is hard to make a sound whereas the clarinet has horrific fingering patterns), Cello, Percussion (depending on what type?) Bassoon, Trombone, Irish pipes, Guitar, Piano Accordion, Tuba/Sousaphone, Double Bass, Saxophone, concertina, Digeridoo, Scottish Bagpipes, Flute, Ukelele, recorder, Kazoo.