Drummer here. Easiest?, imo, drums - for general level of playing. When I used to teach I could have a student playing beat#1, which you've heard in the majority of your favourite songs, within a minute or two, and they'll sound solid. I don't think you can do that as well or fast with another instrument.
That said, at top tier level of performance, elite of the elite, also the drums. It's cute when guitarist say the guitar, we expect that, but try using all four limbs, each limb doing completely independent rhythms, at high tempo, using odd times, while trying to reach all these various instruments, and not have it fall apart. Never mind the insane physicality it takes (they've done studies on this, it's wild). Watch a Terry Bozzio, or Vinnie Colaiuta, Dave Weckl, Trilok Gurtu, any of the Frank Zappa drummers, etc, at their peak, and it defies what's possible.
Guitar, Piano, Violin, etc, all use two limbs, with their playing area well within reach. I'm not saying these instruments have it easy, or unique hurdles specific to their instrument, but for the reasons I stated, I think it's the drum set.
