This is a difficult question because both characters engage in words and deeds that are equally brash.
At first glance, it would be easy to say Clytemnestra because she is having sexual relations with a man (Aegisthus) who is not her husband and because she lures her husband to his death.
On the other hand, the whole reason that Clytemnestra wants to get Agamemnon to walk upon the crimson tapestries is so that he will commit a brash act (the Greeks call such brashness hubris). Walking on these tapestries is something that Agamemnon says Priam would have done if he had been victor in the Trojan War. Most importantly, walking on these tapestries is an honor that Agamemnon knows is fit for gods, not human beings ("That's how we honour gods, not human beings"; Ian Johnston translation). Thus, Agamemnon tells his wife to "honour me as a man, not as a god".
Despite Agamemnon's knowledge that walking on the tapestries will put him on a level with a barbarian king (Priam) and the gods, Agamemnon still walks upon the tapestries. In doing so, he worries that one of the gods may be envious and kill him: "...may no distant god catch sight of me / and, for envy, strike me down."
So, while it would be easy to say that Clytemnestra is the most brash character in the play, I would not be so quick to dismiss Agamemnon as a candidate for the title of most brash. From a modern perspective, it is difficult for us to see how walking on the crimson tapestries would be considered brash, but from the perspective of Aeschylus' Athenian audience this would have been a very brash act; and, of course, even Agamemnon himself knows the danger of this action.