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.

Juron arc (8)

Juron is a class S ESP working for Andromeda union.

His goal is simple: Destroy United Sol.

This arc has been completed.

[Juron arc] [1] [The mad and the crazy] [9604]

I personally think Juron and Sae get along well.

[Juron Arc] [2] [The Knights] [9608]

The Knights don't have their own arc.

This is one of few stories dedicated to them.

[Juron arc] [3] [The Grand Agenda] [9614]

[Juron arc] [4] [The council] [9616]

[Juron arc] [5] [New enemies] [9633]

[Juron arc] [6] [Stormchaser] [9649]

[Juron arc] [7] [3rd invasion] [9652]

Parallel story is [Kain and Suu arc] [13] [3rd invasion] [9652].

Just as Kain is running out of options, so is Juron. He, too, is running out of everything.

[Juron arc] [8] [Bright Red] [9662]

Parallel story is [Kain and Suu arc] [16] [Bright Red] [9662].

This story concludes this arc.