Sauron: Didn't part with it of his own free will. Had it cut from his finger. Isildur: Didn't destroy it and only parted with it when killed by orcs. Deagol: Didn't seem willing to part with it and only parted with it when murdered by Smeagol. Smeagol*: Lost it unwillingly when it slipped off his finger. Bilbo: Parted with it willingly. Frodo*: Did NOT part with it willingly by had it taken from him, first by Sam, and later by Smeagol. Sam: Parted with it willingly. * Smeagol and Frodo, alone of the above, lost the Ring once but got it back later. However, neither parted with it willingly the second time either. BTW, I just realized, Bombadil and Gandalf had it briefly too, but that was of very negligible time.