From a healers perspective that has been in progression groups with all three tank classes. It does not matter, they are all fine. Just like with healerís classes, player is more important than class. Personally my two favorite tank classes to heal are shadow and vanguard, but right now we are running with two Guardian tanks with no trouble, so it does not matter.
As the OP example of HM EC, I love healing the Kephess fight with shadow tanks. I can pretty much set back and relax as a healer on this fight with two well played Shadows. Not difficult with a vanguard or guardian either, but with a shadow, I have no worries.
However, it may have something to do with me completing so much endgame content with one particular shadow. I know when he needs healing and I know when I can concentrate on the rest of the group. Have a similar understand of our Vanguard too, where I am still trying to figure out guardians. I also have the added advantage with the shadow of having played a dps shadow, so I have a limited firsthand knowledge of some of the shadow tankís abilities.
I will say the smuggler sawbones tank is the best at pulling and holding aggro for those far range and hidden targets. Like in the room with 2 cores in HM TfB.