[Legolas]

The elves are definitely not "perfectly good" - I've posted on this in another thread today too.

Tolkien says this in his letters:

Some reviewers have called the whole thing simple-minded, just a plain fight between Good and Evil, with all the good just good, and the bad just bad. Pardonable, perhaps (though at least Boromir has been overlooked) in people in a hurry, and with only a fragment to read, and, of course, without the earlier written but unpublished Elvish histories. But the Elves are not wholly good or in the right. Not so much because they had flirted with Sauron; as because with or without his assistance they were 'embalmers'. They wanted to have their cake and eat it: to live in the mortal historical Middle-earth because they had become fond of it (and perhaps because they there had the advantages of a superior caste), and so tried to stop its change and history, stop its growth, keep it as a pleasaunce, even largely a desert, where they could be 'artists' – and they were overburdened with sadness and nostalgic regret. In their way the Men of Gondor were similar: a withering people whose only 'hallows' were their tombs. But in any case this is a tale about a war, and if war is allowed (at least as a topic and a setting) it is not much good complaining that all the people on one side are against those on the other.

Orcs could rebel against Sauron, but it does seem that they have some inherent obligation to Melkor (and evil, consequently). This quote implies that:

Sauron is just another (if greater) agent. Orcs can rebel against him without losing their own irremedable allegiance to evil (Morgoth).

This quote gives orcs hope, though:

Melkor had the right to exist, and the right to act and use his powers. Manwe had the authority to rule and to order the world, so far as he could, for the well-being of the Eruhíni; but if Melkor would repent and return to the allegiance of Eru, he must be given his freedom again. He could not be enslaved, or denied his part. The office of the Elder King was to retain all his subjects in the allegiance of Eru, or to bring them back to it, and in that allegiance to leave them free.

If Melkor could repent and resume his role in the world, surely an orc could repent and turn from evil. There seems to be another quote about this somewhere, more specific to orcs - I'll have to keep thinking.

Also, from Letter No. 131:

The Enemy in successive forms is always 'naturally' concerned with sheer Domination, and so the Lord of magic and machines; but the problem: that this frightful evil can and does arise from an apparently good root, the desire to benefit the world and others - speedily and according to the benefactor's own plans - is a recurrent motive.

The boldface is especially worth considering when thinking about this subject.

All of the characters in Tolkien have at least -ONE- good thing about them, even if it is just singleness of purpose (a quality admired in allies but not foes).

Quite right - all beings 'are' by the grace of Eru, and coming from him and taking part in the music; being a part - just simply 'being,' and as such, they all have some ultimate good in them.