what is the best defensive formation?

[Potemy]

10-0-0 is not a possible formation, and even if it were, I believe they would eventually break through with no resistance further up the pitch in midfield. You'll also be playing an illegal formation, at least due to no striker.

Seriously, though, I would go for less defenders and more DMs. DMs will cover more squares zonally than their defensive counterparts; they help to contain Ms as well. Central players also cover more than wide players. I played 3w-3n-3w-0-1 and managed to bring a Cup tie with a team two levels higher to extra time after two legs:

---------ST----------
----------------------
ML-------MC-------MR
----DMC-DMC-DMC---
DL-------DC-------DR

Tactics were hard defending/defending, through the middle, own half, hard, long, zonal, counter attacks, no offside traps. Blue arrows on all six midfielders.

[Macquarie Uni]

I play the same defensive format

[namir9]

shes level 6 with 36.5 quality
and mine lvl 3 with 30.5 quality
btw this is her team
and this is my team