If you don't "think" one of your pens is "best", even being free as you are to define "best" in whatever way suits you, then "I don't" seems perfectly adequate to me, with or without the explanation of why you don't think one of your pens is best. It's the same as if someone asked "Do you want white cake or chocolate cake?" when you're not in the mood for cake. You can say, "No, thanks." or you can explain that you had 2 slices at lunch, are at that very moment contemplating the unfortunate fact that unbuttoning your trousers in public is socially unacceptable, and are loathe to eat more cake on the off chance that it would compound your difficulties.
Bookmarks