In response to what xXxKiraraxXx wrote, the bassoon can actually cover almost 4 octaves, especially if a low A extension is added. Also, since when can you get a playable reed for any less that $8? Finally, the standard German system has only 17 keys (but a few more holes). Out of the instruments I play, I would put the in order like thins (1=most difficult)
1. Guitar (v. hard to progress and become a really good player)
2. Violin (OMG OMG THE TUNING DRIVES ME MAD!!!)
3. Percussion (JESUS WILL PEOPLE STOP SAYING ITS EASY!!! You have to count bars for 10 mins and come in with the most noticable instrument at exactly the right time at the right dynamic or the conductor stops the whole orchestra to shout at you. If you don't think it is technically difficult just watch the percussion final of BBC Young musician (one of my friends was on it!))
4. Bassoon (Most people overestimate the difficulty just because very few people play it. I got to grade 6 in 6 months without having played a wind instrument before)
5. Piano (anyone can bang on it, although actually it takes something special to create the near perfect tone you aim for-yes you can change your tone on a piano, just not as obviously as with other instruments)
THEY ARE ALL DIFFICULT IN THEIR OWN WAY!