Here are my two cents for the discussion:
the
Ruling Ring was off of his finger and safely kept in
Barad-dur somewhere
This fits into my ways of thinking, that
the One Ring had immense power, not only when it was on the finger of
Sauron. I think that the word "possession" here means only "possession", wherever the thing, i.e.
the Ring actually was. As long as it was guarded in Sauron's house, he could use his power, because from there it kind of affected him, strengened him.
Maybe its effect is related to proximity. So when Sauron was on
Numenor, far away from the Ring, his power was lightly decreased, else he could have taken over the island sooner, who knows. And when he actually laid his hands on the metal, his power was strongest.
And even if he was only an
evil spirit without physical body, he still had the Ring enough to him, to use its power.
As for the other Rings... Obviously they all gave the person that wore them great power. And some of that power turned into evil. (As for
the Nine and the Seven.)
The Three, however, were given to the
Elves right after being made. One to
Galadriel, two to
Gil-Galad, who gave one to
Elrond on the instant, and one to
Cirdan later, by which way it came to
Gandalf. But that's not the point.
My point is, to bring a new twist in this discussion, that even the Three had the power to be evil. Though they were never used to that purpose, all of them could have been corrupted easily, had they not been hidden from Sauron and had they not been made without his awareness.
The Ring of Earth: Earth is the mother of life. Flowers and other
plants grow on it, nourishing all that live. But it also has immens destructive power. It quakes, it dries out so everything living on it dies, because there is nothing left to eat.
The Ring of Water: Water is as essential to life as is earth, but it also comes from the sea in great storms and leaves the shores in ruin, it comes in floods, so everything drowns.
The Ring of Fire: Here, the main power may actually be destructive, but it also contains a good side. Fire burns, and thus purifies. Silver for example. It leaves the earth burnt, black and desolate, but from the ashes comes new life, stronger as before, as it had to fight its way through the ashen layers. It is a fire, that loves, that drives passion, that creates new things.
So like always, good and evil lie very close to each other, and without
the one, the other cannot exist. (That is, why with the destruction of the One Ring, the Three also lost their power, having lost their evil counterpart.)