[Farael]

Now, I would not want to ruin this spectacular debate with a bit of speculation (and no irony there, you guys are interesting to read) but are we forgetting one little detail?

Kuruharan you state that Sauron's armies would defeat the Ringlord's but... when Samwise a hobbit, and not the greatest one at that, was clutching the ring, Snaga (I believe it was him the orc who was running down the stairs) saw him as a huge, threatening figure. How would the other minions of Sauron see a more powerful ringlord actively using the ring?

I don't know about being able to gather enough troops for this difference to be meaningful, but if the ringlord had enough time to build up a strong enough army, and possibly choose the terrain where the fight would take place (after all, waiting only benefits the new ringlord, thus odds are Sauron would not wait for too long before launching his attack) it is possible that his army would have defeated Sauron's.

Now, in this scenario there is the issue of what would happen if Sauron himself decided to show up for the party. That'd be an interesting scenario, as I'm sure Sauron's minions fear him twice as much as they'd fear the ringlord... what would happen then?

I do not know, but the way I interpret Tolkien's comment is that, in a one-on-one battle, Sauron would win against all save perhaps Gandalf. At the same time, the characters in the book are talking about another scenario, what would happen if a strong-enough ringlord decided to battle his armies against Sauron's?

And keep in mind that none of the characters talk about certainty... so perhaps it is possible for them to defeat Sauron's armies, and effectively usurp Sauron's place (although sooner or later Sauron would arise again since the ring was still both existant and "unmastered")* but only Gandalf could defeat Sauron himself.

*Taking into account the previous comment that mastering the ring would be to break the link between it and Sauron, something only Gandalf could have done. This is unlike wielding, which would be using it without achieving mastery.
Just my cent and a half (short of two cents)