i think kvitova obviously and also keys, she seems so scary when she's in the zone
Seeing her tears had me cackling. I bet she had a similar reaction when WADA ended her.I have never seen anything like the Serena peak at the 2012 Olympics. A lot of other WTA players tend to peak just in terms of their big ground strokes landing in, but Serena had the huge unplayable serve, backed up by the fearsome groundstrokes, so that's the scariest peak (although I don't really find it scary myself, must be scary for her opponents though).
Still, honestly, the answer must be Serena. The scariest peak does not mean she can ever reach it again - although she might come close for a set or a match.