[Legate of Amon Lanc]

If I can meddle in, with the Gandalf-thing, that's exactly the "ultimate good" in its, well, real basis, or how should I call it. You can define Ultimate Good (like a, let's say, concept) as something that does not harm anyone. That can be Radagast, who in fact, did not harm anyone: he stood out of the other's way, did not desire for anything, did not hunger for power... even (as far as we know, but let's presume that it is like that, at least in my mind that's my image of him) whatever he did just for himself, did not harm the others. You see: you can have a person who does not hunger for power or anything else, but indirectly harms someone: let's say a rich man sitting in his palace, who does not wilfully harm anyone, but does not care that people out there starve. I think Radagast was not that far to just "not care" of anyone, but still he could have been a lot more active - he surely spent quite a lot of his time just by being idle. Had he not been given his mission, it won't be considered as "failure". The trouble is that he could also do something somewhere else. So just "meaning no harm" is not close enough to the "Ultimate Good", no way.
The closest to Ultimate Good is, therefore, shown for me in Gandalf. He gave his hand wherever he could, but he was not aggressive. He had these moments of "righteous anger", but I don't recall a moment when this anger would be wilful and evil-doing. It was always perfectly in place, helping in the circumstances. And, again, it was not meant to harm anyone. Gandalf heeded his own words about taking life (FotR II). Gandalf did not have a permanent home, so that he was not bound by anything - unlike Saruman (or even Radagast). Wherever was need, he could go. That's one part of the "ultimate devotion", not as easy as it seems when you just skim-read it. I also don't recall that he would ever do anything just for himself. Except for smoking a pipe, and that, as well as rare and short "regenerating" visits of the Shire, I think was the most comfort he ever gave to himself, and as he himself said, he only did it to relax and refresh because, as any living being, he needed it. In other words, he did this to prevent himself becoming a workoholic. (And not to speak of that even his visits of the Shire turned to be important for his mission - what do I say - even the most important of all! Isn't that wonderful? And now who says there was no power behind the events in LotR, look at this!)

And, to somewhat "step out" and answer (or react) to the very first post. I think, as it was said here, that Middle-Earth is influenced by "outer good power". Eru is not mentioned as giving the Elves or the Númenoreans any divine direction or law, however, it just may be that something like this is just manifested differently here - like many other things. Speaking of the divine guidance in general: The fact that it is not seen does not mean it is not there. And I'm convinced that sometimes you can indeed see it working behind. As for the counter-Morgoth figure that you say we are lacking, do we need it? Maybe it will make sense in some sort of dualistic universe which (and also for this reason) Middle-Earth surely is not. Even in Christianity, as you brought it up, the power of God does not show in having one Ultimately Good figure representing it and fighting all the battle for Good. Before you interpretate it wrong, see what I mean: Even Jesus, from the "wordly" point of view, does very little in his life on Earth. He does not go and persuade the Roman Emperor to lower taxes, free slaves or whatever, or claim his throne (and based on what we know, something like that is what many of the Israelites expected the Messiah to do!). Although, from some point of view, yes, God directs everything - but as in Tolkien's works, most of it cannot be seen (cf. f.ex. Luke 17,20n). And even what is seen, is often hidden from the eyes of those who do not believe (Matthew 13,11 and parallel). The Kingdom of Jesus Christ "is not of this world" (John 18,36), it is more like the "kingdom" of - yes - Gandalf. Jesus rejects Satan's offer of rulership of the world (Matthew 4 and parallel), which is from my point of view the exact image of what Saruman did not do - speaking metaphorically, he accepted this offer! But the humble characters are the ones who make the real difference. Big kingdoms rise and fall - Maedhros' Union, Gondolin, Doriath, even Númenor - but the true - yes, if I can use that name, "Kingdom of Eru" remains and builts its way through Bilbos and Frodos and Gandalfs and little changes that cannot be seen, but are.