[Boromir88]

Originally Posted by The Mouth of Sauron

Denethor ? Although he was driven mad by what he saw with the Palantir, he was "too great to be subdued" according to Gandalf.

Granted one could say irrelevant since Denethor was in fact dead by the time Aragorn leads his host to the Black Gate, but I have a touch of pity for the man.

I agree about Denethor, but in a different way. He was still "mail-clad" during the Siege of Gondor, but I wouldn't call him a knight in the sense of a warrior. Since, Denethor makes clear to Gandalf he believes as Gondor's leader he would be more effective doing so from his seat (as Sauron sits back in his tower to "lead."):

Denethor laughed bitterly. "Nay, not yet, Master Peregrin! He will not come save only to triumph over me when all is won. He uses others as his weapons. So do all great lords, if they are wise, Master Halfling. Or why should I sit here in my tower and think, and watch, and wait, spending even my sons? For I can still wield a brand."~The Siege of Gondor

Having said this, Denethor still had a strong and capable will (as proven in his ability to use the palantir). He was in fact the leader of Sauron's most hated enemy, and as such, Sauron feared him. Unfinished Tales (The Palantiri) notes that Sauron had no servant with a strong enough will to match Denethor's, and thus it is why Sauron himself has to use the palantir. Once Sauron discovers Denethor is using the palantir in Minas Tirith, he discovers an opportunity, as Gandalf puts it:

"Even in the heart of our stronghold the Enemy has power to strike us: for his will it is that is at work."~The Pyre of Denethor

So dead by this time in the story, and not a front-line leader. But for all his faults, Denethor arguably had the strongest will of the men of Gondor, and was a big enough concern to Sauron that Sauron himself had to use the palantir to make his "strike at the heart" of Minas Tirith.

Originally Posted by Glorthelion

The king of dale, Thranduil and Dain deserve mention too although in my knowledge Thrandiul never participated in any battle whatsover. The Hobbit never explicitly mentioned that Thranduil was involved in the battle of Five Armies, although he did lead the elves there. But him having led the elves there I am inclined to think that this implies he did participate in the battle.

Regarding Thranduil, I agree that since he's noted as leading the Elven host at the battle of Five Armies he in fact fought in it. As I mentioned with Denethor above, there were certainly leaders who did not lead their armies at the front, but preferred using others (namely Sauron and Denethor). However, rarely would Denethor and Sauron be mentioned as "leading" during a battle. In Gondor's case, it's normally whoever the lord was of the certain fiefdom.

For example, Imrahil is mentioned when Tolkien wants to talk about the knights of Dol Amroth during the Siege of Gondor. Or Forlong, when it is the men he brought from Lossarnach. So, the fact that Thranduil is the Elven-King, and is the leader of the Elven host at the Battle of Five Armies, is I think sufficient enough to conclude he was present and fought in the battle. If not, then he definitely fought during the Last Alliance. His father, Oropher, was the leader of the Mirkwood elves at the time, but dies during the battle and Thranduil is specifically mentioned as leading the surviving Elves back to Mirkwood. (Basically, I'm just trying to get at, Thranduil has been an established and present name in battles).