Radagast the Brown??

[mormegil]

Two quotes on the blue wizards

In 1954 he was uncertain about the Blue Wizards, but in a letter of 1958 he says of them "I fear that they failed, as Saruman did, thought doubtless in different ways; and I suspect they were founders or beginners of secret cults and 'magic' traditions".

--The Letters of J.R.R. Tolkien, p.280

However, in The Peoples of Middle-Earth they get a happier ending; they "must have had great influence on the history of the Second Age and Third Age in weakening and disarraying the forces of [the] East".

-- The Peoples of Middle-Earth, p.385

So it seems that Tolkien himself was divided. But it should not be forgotten that he said in unfinished tales

of all the Istari, one only remained faithful

Even though Radagast can be deemed to failure, it has been suggested (although I'm not sure if Tolkien himself said so explicitly) that Radagast was responsible for the Eagles' coming at certain appropriate times (such as the Battle of the Morannon and Five Armies).

Rather I think Gwaihir was responsible for these actions and Radagast had little to do with the actually movement of deep events in Middle-earth.