Water 7 Saga hands down. It holds two of the best story arcs of One Piece (Water 7 and Enies Lobby, IMO they're THE two best arcs of One Piece), plus a decent introduction short arc (many may argue that Long Ring Long Land is a below average arc, but it's alright considering only the manga) and one of the best short "postlude" arcs of all time (Post-Enies Lobby).
On the other hand, the Summit War Saga is a bit inconsistent. It has another top short arc (Sabaody), a so-so arc (Amazon Lily), a pretty solid arc (Impel Down) and maybe the best conclusion short arc (Post-War Arc). While Marineford itself is a fan favorite, IMO we can see some flaws that would become more evident in later arcs (e.g.: Wano, Oda is unable to conduct a large-scale conflict).
Keep in mind that the things said above are only my opinion and I'm not the "know-it-all".