Yeah, I would lean towards her myself. Sif has had more screen time, with appearances in two films and a few episodes of AoS, but Valkyrie was pretty decent when she was in action. What's more, while I normally loathe to use character statements in these types of arguments, based on how Thor talked them up, the Valkyries were a seriously powerful, elite force.
Incidentally, Sif was absent from the latest Thor film because of scheduling issues between Jaimie Alexander and Marvel. At least, based on an interview she had. Apparently, she was busy filming season 3 of Blindspot when they phoned her up, and she couldn't come up with a compromise on short notice.