Define debuffs not scaling down. 840+ Isn't even remotely good for heroic - remember that normal drops 850, and a rule of thumb with all this TF bullshit going on is that the next level of difficulty is tuned for what the previous one drops (so, 850 average).
As for easiest, depends on your setup. Cenarius and Xavius are significantly more annoying than the five previous bosses, but it all comes down to general raid strength;
Ursoc is pure throughput with zero real mechanics. Got a lot of dps/healing, just smash him.
Dragons requires you to be smart about spreading/standing on flowers and not taking extra damage as it's a quite long fight that'll exhaust the healers. Probably the easiest of the lot.
Ilgynoth requires people to be good at prioritising targets and switching, and apart from that very strong snap healing (two hits in a row from the eye beam = dead if you're not topped), with minor dps checks (which can partly be carried by bloodlusting the start of the second P1). I'd go with second easiest here.
Spider boss is just all kinds of nope. One idiot puts slime too close to a web or tornado spawn, and you're in for a quick wipe. One tornado is too close to slime and you better hope it doesn't move, or you're getting pelted again. Likewise, running over the web with one person being able to kill an entire group by touching an egg = bad for most guilds that needs advice on which order to go on heroic.
So, in the end, IMO: Nythendra>Dragons>Ilgynoth>Elerethe>Cenarius>Xavius. Slot in Ursoc anywhere between the first 4 depending on your raids gear level.
8 minute enrage, 1660M health in 30 man. Assuming 6 healers, 2 tanks, that means 1660M for 22 dps. This comes out at 75.4M damage per dps over 8 minutes, which comes out at 157K dps per DPS player - or about 150K when adding in tanks.