A couple of thoughts:
Radagast: I think the best yardstick to measure Radagast by is Saruman. Given the contempt with which Saruman holds Radagast, I think we can safely strike him from the list. I can imagine Saruman creating a mental or actual list of those who could contend with him. Whether by inclination or capability, it looks like Saruman does not consider Radagast a contender. (The irony, of course, is that Radagast appears to be following the same path as Bombadil, over whom the One Ring has no hold.)
I think we are missing one clear test for membership on that list — the individuals selected by the One Ring for temptation. It does not reach out indiscriminately. It did not tempt everyone in the Fellowship. It reaches out for Gandalf, Aragorn, Boromir, Frodo, and, eventually, Sam — asking them to consider what they would do were they to claim the Ring for themselves. The One Ring does not ask Pippin, Merry, Gimli, or Legolas to consider such things.
The others who actively refuse the Ring (or openly discuss it) are Dethenor and Faramir, Galadriel, and Elrond. It doesn’t not seem to bother Celeborn or, for that matter, Glorfindel (although it is possible he is just being discrete). And, of course, Sméagol/Gollum is somewhere in the mix.
And, standing head and shoulders above them all (metaphorically speaking) is Bilbo — the one person to willingly give it up.
From this vantage point, it appears the One Ring is going through a kind of triage list and focusing its attention on those who are most likely to claim it and/or wield it. To take Sam as an example, he is not noticed by the Ring at the start of the story. He, like Frodo, has grown by the end of the journey — raising himself in the estimation of the One Ring and becoming worthy of temptation.