They are by the same makers.
So, I just don't have a choice as they were all great.
That wasn't always the case. Bethesda took over the place Black Isle used to have ( being the developer). In my opinion, this caused the great downfall in the Fallout series, since otherwise, they woulden't have used the same engine Bethesda made for oblivion.
I'd have to go with Oblivion .