i Cant really understand.... what's the best soul gem for getting the maximum stats when encahintg a piece of armor or weapons, and the black soul gem is the strongest usable or it's made only for the vampirism cure ritual?

Grand Soul Gem with a grand soul inside, or Black Soul Gem, with a grand, or npc (black) soul inside.

As Atkara said. Either a Grand or Black soul gem with the highest possible soul inside.

Mammoths, Dragon Priests, Deathlord Archer w/ U-Force, Falmer Shadowmasters (with Falmer mask), and Falmer Warmongers all give Grand Souls.