Alternated the first couple chapters between Jake and Leon, then switched to completing Leon first. Then finished up Jake second, straight through the Chris and Ada campaigns afterwards. No claim as to best order, just went by whim.
Helena up until China, First part of Chris in Chinatown until Edonia up to where Fin blows up the AA guns, then played as Sherry for the first two chapters. Realizing that no matter what order I play it in the story still is not making much since to to a lack of in-game files, finished up as Helena, then Sherry/Jake, now going to finish the rest of the game as Chris. Mercs is more fun then the campaign overall and have been playing more of that lately(Jake, Helena and Leon are my good characters - Chris takes forever to do anything and Sherry while having almost the same armament as Jake has what seems to be the weakest melee attacks in the entire game...).