Easiest to hardest. Gorefiend and Mannoroth weren't necessarily the hardest bosses. It's just that they require your whole raid to focus, and are quite long.
My favourite boss of the tier was Mannoroth. You could tell that you made progress with each and every attempt. One of the best encounters they've ever made in my opinion. Overall Blizzard did a great job with HFC.
1. Hellfire Assault
2. Kilrogg
3. Komrok
4. Iron Reaver
5. Socrethar
6. Council
7. Fel Lord
8. Iskar
9. Tyrant
10. Archimonde
11. Xhul'Horac
12. Mannoroth
13. Gorefiend
I agree. Xhul is more of a "oops I got stepped on and blew up the raid" kind of fight. Easy to maneuver. Gorefiend was just miles ahead in difficulty compared to the first five and some of the later ones like iskar or socrethar. The amount of coordination it required was pretty decent but probably not the hardest coordination in the instance. The gear check wasn't even that brutal, obviously not speaking for those world first guilds, but if you were able to smoothly handle each mechanic before each feast you were solid.
<SPK> US 8th Xavius Mythic - US 10th Helya Mythic - US 8th Gul'dan Mythic - US 9th Kil'jaeden Mythic - US 12th Argus Mythic
7/7M Emerald Nightmare - 3/3M Trial of Valor - 10/10M Nighthold - 9/9M Tomb of Sargeras - 11/11M Antorus the Burning Throne
World first heroic Gul'dan. Almost Wildstar hardcore status.
First 5 bosses are not soo hard,pugged em without using voice chat.