it hasn't been long since I started playing a tank. I'm currently leveling a PT pyrotech now, that I'm gonna try to tank with later, to see how it is. But I must say, the easier one to play of the other 2 for a total noob is the Shadow/assassin, there is no question about it. Perhaps the PT/Vanguard is the easiest of all, but I can't say... yet. But others here have said it is.
and by easy to play, I don't mean master, I mean so it's ok for running hard mode FP's and story mode Ops.
If you ask me, of the 3 tank types, juggernaut/guardian looks most heroic
But is hardest to play.
So, quick fix: PT/Vanguard, moderate: Assassin/Shadow, slow: Juggernaut/Guardian
For PvP I absolutely recommend the Juggernaut/Guardian, purely for the mobility and intercede/guardian leap. Huttball isn't the same without one.