Don't know where to begin?

While ultimately it doesn't matter which arc you begin with, Cecil arc is recommended if you want my recommendation.

And then Masu arc + Shattered union arc. 

Feel free to visit the lore section as well which holds stories of its own.

Masu arc (9)

This is Masu's story. This arc is closely related to Cecil arc.

This arc is completed.

[Masu arc] [1] [Dandelion] [8618]

Masu arc is about a guy named Masu and much later in this arc, his son will take over. Early stories of this arc are closely related to Cecil arc.

[Masu arc] [2] [Masu the sage] [8631]

This story is where he gains the fame and it is one of few stories that shed light on the Bau from inside.

[Masu arc] [3] [3Ra Syndicate] [8625]

This is actually a side story and is one of very few that deal with the Ra clan.

[Masu arc] [4] [Remorse] [8739]

[Masu arc] [5] [Storm] [8921]

[Masu arc] [6] [The Dietrich] [9012]

[Masu arc] [7] [Wrong legacy] [9599]

[Masu arc] [8] [Dr Anna] [9611]

This story covers a minor character in Dr. Anna.

While not related plot wise to this story, it is recommended to read [Sae arc] [2] [Excessive Sleep Syndrome] [96xx].

[Masu arc] [9] [Demise of a servant] [9649]

This is the final story of this arc.