It looks like we have an actual in game system to discuss, albeit in very general terms. Here are my thoughts on each choice.
Living Target: You are best at drawing your enemy's attention away from allies.
Depending how difficult it is to keep aggro and how many enemies a team may be fighting at a time (a few like CO or many like CoX) this could be the go to mastery for team play. Likely not useful for solo play.
Bruiser: Your attacks help debilitate foes over time.
If debilitate means "lowers the damage of", I like this one a lot. Good for grouping and good for soloing. If debilitate means "lowers the resistance of", its still good for both but maybe not as good for tanking imho.
Battle Leader: When your allies focus on you, their attention improves your ability to defend them.
We already have a clarification on this one, awesome! When allies have you targeted (either for buffing or to attack your target), they take less damage and you hold aggro better (based on mastery tier). IMHO this may be the best tanking mastery, but not useful solo.
Ruggedness: You can shrug off more damage.
I could see this one being the standard while leveling, both in groups and solo. If our damage is lower (like tank damage often is in other games), better survivability becomes more valuable. Alternately, If late game tanking is all about surviving big hits, I could also see this being an important end game mastery.
Protector: Your team strengthens your resolve.
I asked in the mastery discussion thread what this means. If resolve means "increased resistance to crowd control effects" then this may be a situational, but powerful, choice for grouping but with no solo use.
EDIT -> Here is a clarification that this has nothing to do with resistances or crowd control effects.
What do you think?