T-Dog gets a pass because he was never really given a chance to act on the show. But I thoughts the actors who played Shane and Dale were particularly terrible. Over the top, cheesy, scene chewing, etc. Dale in particular had this one default wide-eyed, mouth agape look that made me want to punch him in the face.
Carl is bad, but he's a kid and most kid actors suck.
Of the group, I think the actors who play Glenn and Merle are the best. The actor who plays Merle is surprisingly great. He can go from being truly disgusting and menacing to having a kind of kicked puppy dog look. He's fantastic. And the actress who plays Carol has gotten much much better I think. Conversely, I thought the actress who played Lori got progressively worse. She too had that default eyes-bulging look all the time.
But overall, the Walking Dead is not a show you watch for good acting.