Which province do you think is the best at defending against an invasion? and which province could form the strongest invading army?

Black Marsh has my vote for best defense.

Yep, Black Marsh. Elsweyr and Valenwood are probably also well off, due to their harsh terrain. On the other hand, none of those are unified enough to launch a coordinated defense or counterattack, leaving them to resort to guerrilla warfare.

Poisonous swamps do tend to create a pretty hefty barrier.

Valenwood is also pretty defensible- the Bosmer can climb trees like none other, and depending on the army, if you have heavy Imperial Legionaries marching through, the Bosmer can ambush you, pick you off from the trees, etc.