Or Mr. Thicknose, he's heard a lot of things and remembers a lot over his long life. I'd say Grandpa Longneck and Mr. Thicknose may be about equal, with each having strong areas where they are smartest in and having a lot overlap also.
Of the residents of the Great Valley it would likely be Grandpa or Grandma longneck would be the strongest since they are larger then threehorn, though since they don't exercise any Threehorn may be stronger and very likely has more endurance. Of the characters who have ever appeared on lbt, though he has only 2 cameo appearances with 1 being non speaking Sue's mate would likely be the strongest overall, though we are unlikely to ever see him again so he may not count and isn't a resident of the Great Valley.