In terms of being ultra-faithful to the manga, it would have to be Act 6 I suppose, but if I were in charge of Crystal and had some more creative freedom, I'd have made it Act 4: I like the idea of Rei meeting and befriending Mako before the other girls meet her (especially because it would break up the monotony of having Usagi befriend a new girl and she turns out to be a Sailor Guardian each episode), and I'd like to delay the Masquerade chapter a bit so that Mako could attend it, too.