Hybrid sin tank: I find it to be just as tanky as any other tank, but it has the added bonus of being able to actually do damage as compared to the others.
Operative healer: Most mobile healer, pretty much nothing else needs to be said :3
Madness Sorc: Terrific single target damage, has the ability to cleanse ccs if the healer in incapacitated, and also off heal. Do not underestimate Extricate :3
The last spot is up for grabs, I think a carnage mara or madness assassin could fill the roles just as well in dealing very strong damage, the mara being burstier, with the sin putting up more long term pressure.