Define "best", please.
Most flamboyant, most "efficient" on long-run, the one that produced most important kings, the one that had most "kinds" rulers...etc.? (Of course, non of these are "best", but litteraly what I just said).
As reality, for some reason, doesn't gave points to ruling dynasties as in Crusader Kings, it's hard to hierarchise.
Personally, I would think Robertians/Capetians, as they managed to hold firmly Western Francia (including the part where they held Neustria proper) managed to form a dynasty most representative of medieval era. Having skilled and/or important rulers every 2 generation helps, granted.