Mhairi is that person who you get for like ten minutes at the beginning of Dragon Age: Awakening. She's fairly forgettable as a character, but she is technically a female tank. In other words, it's just Mr. Carny being thorough.

I'd have to go for either Aveline or Cassandra, but I'd lean slightly towards Aveline. Aveline works in conjunction with Isabela on so many levels. I frequently have them in the party together (which actually works, combat-wise) for the banter they have. It is priceless. I'm sure there's a compilation of their funniest moments in Youtube.

Cassandra is also excellent and feels like a really fleshed out character. Shale is also priceless and full of dry, sometimes dark humor, which gets me chuckling. Though as a tank, Shale is far outclassed by Alistair. Shale's best role is to off-tank, letting Alistair take the tanking job, while she proceeds to use her superior offense to great effect. Shale is pretty much best paired with Alistair for gameplay purposes. In fact, why you wouldn't have Alistair in your party is beyond me; he's almost mandatory unless you respec Sten or Oghren to take up defensive duties.

In terms of combat usefulness though, again I'd have to say Aveline. Her array of skills makes her nigh invincible, and with Aveline your only real tanking option in Dragon Age II (Origins has so many warriors you can almost take your pick) Aveline becomes part of the furniture. Cassandra also fills that role for much of the early game, until you get Blackwall. Cassandra is more of an offensive tank, while Blackwall more accurately fulfills your typical defensive tank duties.

I do hope they continue this trend of female tanks though, it's sort of become BioWare's thing. If you extend it to Mass Effect, you could say Ashley Williams fulfills the same stereotype busting role, as the strong Soldier type.