Loghain is pretty obvious.
Leliana technically betrayed Orlais, but unknowingly, and once she DID know, she tried to set it right.
Zevran is also pretty obvious, but the determining factor of whether it remains one-sided or a double-cross is you.
Wynn never really betrays anyone. Yes, she is technically an abomination, but only in "form" (spirit and person in one body), NOT "function" (kill everything)
Ogren I don't see as a traitor, because he's not really loyal to any one thing in particular. If anything he was betrayed, first by Orzammar (to a degree) and then by Branka (wholesale)
Sten is a traitor at first, but due to the idiosyncracies of the Qun (from our perspective), he can potentially NOT be a traitor if you find his lost sword.
Alistair is much like Wynn, in that he's really not a traitor at all. He never actually completed the Templar training (i.e. he never took lyrium), so he can't betray an organization he was never fully part of. And he's also the one who always stands by the Grey Wardens, UNLESS you take on Loghain, in which he betrays the Wardens, but ONLY because you forced his hand.
I know of a very specific circumstance where Alistair does not become a drunk and Loghain is spared, but it's very particular and I don't really care for it in my playthroughs.
Morrigan is a traitor "by default", due to her "look out for number 1" mentality and training under Flemeth. To clarify, she's only loyal to herself and knowledge, everything else be dipped in hot molasses. Frankly, only a Warden who romances her (even more so if they have the god-baby together) has any real shot of influencing that aspect of her.
So, in a metaphysical way, you, the Warden, can have Morrigan betray herself, or at least who she WAS, through the dialogue and interaction.
And of course, Dog is Dog, your ever-faithful friend and companion.