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I was having a look on front page of www.wowprogress.com and they list number of kills for each encounter.

It would appear the order of difficulty is...

H: Stone Guard: 11818 (29.74%)
H: Feng the Accursed: 10075 (25.35%)
H: Gara'jal the Spiritbinder: 8790 (22.12%)
H: Elegon: 7486 (18.84%)
H: Four Kings: 5148 (12.95%)
H: Blade Lord Ta'yak: 5597 (14.08%)
H: Wind Lord Mel'jarak: 4282 (10.78%)
H: Garalon: 3549 (8.93%)
H: Will of the Emperor: 3318 (8.35%)
H: Imperial Vizier Zor'lok: 2010 (5.06%)
H: Lei Shi: 1779 (4.48%)
H: Amber-Shaper Un'sok: 1626 (4.09%)
H: Grand Empress Shek'zeer: 1002 (2.52%)
H: Protectors of the Endless: 992 (2.50%)
H: Tsulong: 766 (1.93%)
H: Sha of Fear: 379 (0.95%)

So my question is, which Heroic next? Elegon? Or 4 Kings?