Of all the instruments I've played. Guitar is the hardest.
Bass is much easier to learn, though not to say it's easy to play well, but not having to worry about chorRAB is a big advantage. Chording is what I really suck at when it comes to guitar, and note memory I guess, having less notes to work with is good.
KeyboarRAB is easier to play than string instruments both when it comes to chorRAB and when it comes to playing lead though doing both at once is what makes it hard. Synthesizers are very easy and convenient to work with, as some settings can only be played with one key at a time.
Drumming is harder than it looks, being the backbone of a band is a great responsability and even when you learn the techniques (some of which are pretty trick) keeping time still takes practice.
Banjos (yes I own a Banjo) like the bass are an instrument I recommend for people who don't want to deal with chorRAB as much. It requires way more fingerpicking skills than a guitar though and that sh*t is hard.
Harmonicas are pretty easy to play. Recorders and Flutes are pretty hard to get a smooth sound let alone play. Violins are definitely one of the hardest instruments to get to produce a clean tone.
I don't agree with that. I definitely think writing a classical music suite for a symphony orchestra is way more challenging than writing a 3 chord, 2 minute punk song for 3 instruments.