I've only done BoT and Conclave on 10 man while I have cleared everything on 25 man.
BoT and Conclave seemed easier on 10 man even on my alt, except maybe for Cho'gall but I guess he depends a lot on raid setup.
Chimaeron took a long time for our healers to figure out on 25 man, but I've heard other guilds having an much easier time on so I guess it depends on how good your healers are.
I would say Conclave, Halfus and Val & Ther are the easiest of the bosses you have left. When you want to go for the easy bosses depends on raid morale and how you like to raid. Do you want to start the raid of with some easy kills and then bang your head against the wall until the time is up, or do you want to kill something easy at the end of the raid to boost morale? Keep in mind though that people might be tired and unfocused toward the end raid so the "easy boss kill - morale boost" might come right back and bite you in the ass.