This is a special mention up ons; I couldn't possibly keep this waiting another moment. EHHH SEEEE post later in the week, I think I've got enough vocaloid-related ones to go for it. In the meantime:
【メドレー】ボカロ25曲をアコギとかでアレンジしてみた【作業用】Osamurai on guitars - twice - is arranged by kaichi to provide an absolutely stunning 40 minute acoustic mix of 25 famous vocaloid songs, back to back with absolutely nothing added. I cannot heap enough praise on this - it's beautiful from start to end and masterfully covered. Listen to it listen to it listen to it.