I think either May or Trudeau had the best performance, which probably means that May was the best. She sounded like a voice of reason most of the time, and she probably exceeded most people's expectations. Don't know if it'll win her any votes, though, but I think it's enough to keep her seat.
Trudeau may have been a little jumpy, but I don't think that hurts him. He's supposed to be the young, passionate guy, so playing into that probably won't hurt him. He had some pretty great lines, and his closing statement was miles better than everyone else's.
Harper was Harper. If I were a Conservative supporter, I'd probably feel reassured. That "Mr. Harper, nobody believes you" line was arguably the closest we had to a zinger, but as I said, it probably won't hurt his Conservative support.
Mulcair was really underwhelming. He never had the right tone, he vanished for long swaths of time, and I feel like he tried to channel Layton but failed miserably. Also, flubbing his closing statement was pretty terrible, considering he was reading it.
Of course, the number of people who watched a debate on a Thursday night in August was probably extremely low, so I'm not sure it'll change much.