For me the real question is not what is the easiest to start on (the harmonica is the easiest, you can sound great immediately, one chord anyway, you can play Red River Valley like a real cowboy in no time at all), but what instrument gives you the least barriers to the next level up? -- not how to be great, but to get competent so you don't hate yourself for a long time. I would certainly say an open-tuned guitar, or a 5-string banjo. You can accompany yourself and if you can learn how to finger pick a little, they sound great very quickly. That doesn't mean you can frail on the banjo, but you can do three chord accompaniments on day 1, and by day 3 you can be doing a few more interesting things with arpeggios, etc. Accordions are not easy to play if you have never played a piano (speaking from recent personal experience)and have no experience in reading music in two clefs. Among the hardest seems to be the clarinet. It is a long, lonely slog to real competence, it makes terrible sounds until very late in the process.

yours, Peter T.