This is a tricky question because instruments can be difficult for very different reasons. I think of it as two completely independent skills--the ability to produce a good tone, and the dexterity to navigate the technical challenges of the standard repertoire.
For tone, instruments like piano are trivially easy. Guitar is fairly easy but there's at least some conscious effort involved. Flutes and single reeds are somewhere in the middle, requiring control of the airstream and embouchure (lip muscles) but still having a good portion of the tone dictated by the instrument. Double reeds and brass involve paradoxically needing to be strong yet relaxed, which requires a lot of practice (and sheer muscle development) to get down. On brass, just playing the right note requires a strong ear and extremely precise lip tension. Strings have an immense amount of nuance that isn't immediately apparent--tiny changes in the position and pressure of the bow make huge differences, and it takes years to start to sound good.
Dexterity is harder to rank because composers will write things that are as difficult as they can get away with. For example, saxophone fingering is very easy, so saxophonists are asked to play very fast and complex lines in response. So is sax technique easier or harder? There's also the question of genre--strummy campfire guitar is dead simple, but classical guitar gets as difficult as you want.
All of that aside, the best way to determine the relative difficulty of instruments is to look at beginning ensembles: middle school bands and orchestras. If you go to any 6th grade concert, the strings will all sound like cats being tortured, the brass will sound like farts (the horns will additionally be playing mostly wrong notes), but the woodwinds will be doing okay. Move to 8th grade and the top members of the brass sections are starting to sound okay, but most of the strings still sound horrible (and the woodwinds are playing circles around everyone). To me, this shows that woodwinds are objectively easiest, then brass, then strings. (NB: It's not fair to judge double reeds here, because they're usually taken from the best flute/clarinet/sax players, so there's a massive selection bias)
And finally, all of that aside, I identify a third measure of difficulty: competition. Piano is sort of middling in difficulty, having trivial tone production and a wide range of technical demands, but there are so many good pianists that if you want to stand out you have to be absolutely amazing. Rock guitar is very easy, but everyone and their dog is a guitarist, so it's all but impossible to stand out, and the guitarists that are famous got there more for their singing, songwriting, and appearance. But there are so few violists that if you can just play in tune with a decent tone you'll be in high demand.
My overall take:
Hardest to play: Orchestral strings, horn, and double reeds.
Hardest to make money with: Guitar/bass and voice