An ideal western army
 

Lets sat that the "good" guys rohan, gondor, lorien, mirkwood etc. All mustered and gathered their armies in the war of the ring, and prior to this had made armour and all sorts of preparations. Would they be able to defeat mordor, isengard, harad and the easterlings in a big clash? The question is a bit simple and childish. But i always get the impression that mordor (sorry for this sentence) breed and populates for war. What would be the outcome if the good guys did this? Could they take mordor?

No.

For one thing, concerted action by all those you name would have been quite unthinkable. The overriding theme in the Third Age seems to be one of a distancing between Men and the other races.

Even if the possibility of a Grand Army of the West is granted, I can't see it succeeding against Sauron. The Last Alliance with the cream of the crop of the warriors of Men and Elves was barely able to eak out a victory previously.

Had they all mustered against the Land of Mordor, there would still have been other foes attacking from behind. Galadriel was busy overthrowing DolGuldur in Mirkwood, Thranduil's folk joined in the Long Battle under the Trees while Orcs from the Misty Mountains and their Northern realm attacked men and Dwarves, killing Kings Dain and Brand in the Battle of Dale.

If the scenario you imagine is that of a victory of numbers or physical power - no. They just don't have enough people for that, and Sauron just has too many (and a way to breed them real quick).

Sauron gets his troops from the cloners on Kamino. Unlimited supply.

And btw this was done in the 2nd Age but Gonder was tons stronger then and there were more superior Elves. In the end they beseiged Sauron and destroyed his body. Few men if any in Gonder even came close to the likes of Elendil and Isildur, at least physically. I would say maybe yes a few in Gondor, but they would probably either be in the Steward's house which was the purest in Gondor or in some house pretty close. Gondor was a shell of itself, sort of like Ray Lewis is now. They got alot of help from their kinsmen in the North during much of the 2nd half of the 3rd Age. Gondor just did not have the numbers anymore. All that infighting and constant badgering from the south and east. So yea Gondor had some really good men of Numenorean descent who were strong, but they would need a lot more than that against Sauron and the men and orcs who were with him. Remember, the Wise were of the belief if Sauron got the Ring back he'd cover all the lands in darkness. He had the Ring in the 2nd Age and was beat. There's a big power gap then and in the end of the 3rd Age. Also when Numenor was at full strength, the power of the High Men alone was enough to go toe to toe with Sauron. None of his servants could stand against them and he even doubted his fortress could withstand an assault by them.