Keller's all my life. However, I recently discovered Cabot's so when I see that brand I go there now (not widely available in my area). Although we get Kerrygold cheese in my area, I've never seen their butter.
Me three, where baking is concerned. For health concerns, I switched to Smart Balance for spreading. I wish it were unsalted but suspect that if it were, it would have little taste. As Frankenspreads go, it's pretty good.
I'm not too picky with butter, but to support local'ish farmers and good animal welfare dairy farming, I tend to pick Strauss or Organic Valley (of course Organic Valley sources differ from region to region)