What gem for Farron's greatsword?

I've tried googling to find the best gem with no results or confusing results since i'm new to the whole dark souls thing. So I figured i'd just ask here directly. I'm currently 23 dex, 20 str, and not sure where to go with my stats. I really enjoy the moveset so far and was thinking of trying to build around this weapon. Any input would be greatly appreciated.