As others have said, both are thoroughly entertaining on different levels.
Katherine was a big part of the reason I got hooked on Dallas.
End Game was my entree to the series and I then re-watched the season leading up to it. She was right at the centre of things and one of the most memorable things about the entire series at that point for me.
Today, Katherine might have still edged into the lead for me if her death hadn't been a dream. Because of her wishy-washy return and forgettable final scene she lacks the true great ending she originally had, which dilutes her story for me.
Kristin had it all, including the ending. Beyond this, she left more of an impactful legacy on the series as well: not least Christopher. Despite Kristin's background being explained more, I also found her more complex. The psychology of the sibling rivalry with Sue Ellen was fascinating (particularly when you factor Patricia into things). Screwing her sister's husband while gaslighting her seemed far more of a betrayal than Katherine making eyes at Bobby because Sue Ellen and Kristin had been in each others' lives for longer (which is exactly what made the psychology between them endlessly fascinating).
Plus she has the kudos of being the original sharp-shootin' sister-in-law.
So, yes, I'll go with Kristin.