People often make the mistake of assuming that a raid instance follows a linear increase in encounter difficulty. It is also easy to get lost in your particular role and view an encounter's difficulty as gauged solely by your raid function. For example, a melee dps might consider Festergut a cakewalk while the healer is consulting his doctor about carpel tunnel syndrome. Below are my general difficulty rankings for the encounters available thus far in ICC, followed by role specific lists. Reasonable minds might differ and I would love to hear how your list would differ from mine.
From Hardest to Easiest:
General Difficulty:
1.) Professor Putricide
2.) Rotface
3.) Blood Queen Lana'thel
4.) Blood Prince Council
5.) Festergut
6.) Deathbringer Saurfang
7.) Lady Deathwhisper
8.) Airship Battle
9.) Lord Marrowgar
Tanking Difficulty:
1.) Professor Putricide
2.) Lady Deathwhisper
3.) Rotface
4.) Airship Battle
5.) Festergut
6.) Lord Marrowgar
7.) Blood Queen Lana'thel
8.) Blood Prince Council
9.) Deathbringer Suarfang
Healing Difficulty:
1.) Festergut
2.) Blood Queen Lana'thel
3.) Professor Putricide
4.) Rotface
5.) Lady Deathwhisper
6.) Deathbringer Saurfang
7.) Blood Prince Council
8.) Lord Marrowgar
9.) Airship Battle
DPS Difficulty:
1.) Professor Putricide
2.) Rotface
3.) Deathbringer Saurfang
4.) Blood Queen Lana'thel
5.) Lady Deathwhisper
6.) Blood Prince Council
7.) Festergut
8.) Lord Marrowgar
9.) Airship Battle