Discounting all IPs with less then 3 games, but still tough one for me to choose:
I love Fallout 1/2, I think 3 is between meh and decent, NV is between decent and fine, but 4 is just horrible, and I pretty much expect them to get even worse.
I love Gothic (yes, even 3rd) but there's only 3 of them...this can change in the future, but until then...
...I'd have to go with Might and Magic, since there are 9 of them (well, 10 if we count Legacy), sure, some of them are pretty bad, but there is both quantity and quality in that franchise.