hsreplay stats confuse me, which number matters for picking a deck?

winrate, but only when the number of games behind it is decent. a deck at 58% over a few hundred games means less than one at 54% over tens of thousands. after that look at the matchups against the decks you actually keep running into. popularity just tells you what's common, not what's good. and the mulligan guide is where the stats really help, it shows which cards win more when you keep them in your opening hand.