For PVE, all tanking and healing classes are viable.
Vanguards/Powertechs are probably slightly better than the others due to HtL/HO but it is not a huge difference.
Commandoes/Mercs are better than Sorc/Sage and Op/Scoundrel but again, the difference between them is small. In general, for PVE play what you enjoy. I would recommend that you pick a different class than your co-healer/co-tank though. So, for example, if your co-healer is a sage, pick a commando or a scoundrel.
For PVP, all tank classes are roughly equal. Healing-wise, I would recommend Sage/Sorc because of their high-mobility.