If you're looking to read the entire WTC series, then you should start with Higurashi. All 8 of the main chapters are available on GOG and Steam, consisting of Onikakushi, Watanagashi, Tatarigoroshi, Himatsubushi, Meakashi, Tsumihoroboshi, Minagoroshi, and Matsuribayashi in that order.
Next is Umineko, which consists of the Question Arcs and Answer Arcs; both are also on GOG and Steam.
Ciconia Phase 1 would be next. There's a whole lot of extra arcs for Higurashi and side stories for Umineko, but those aren't really that important.