Short answer:

  • Raspberries
  • Strawberries
  • Blueberries
  • Blackberries

Long answer:

The fastest way to gain experience points with crops in Farmville is to turn them over as quickly as possible. Blackberries, blueberries, and strawberries can be harvested every four hours and result in 2 XP each (1 for plowing + 1 for planting). Raspberries offer the same XP, but must be harvested every 2 hours to get the same results (1 for plowing + 0 for planting).

Note: When available, Super Berries are the XP crop of choice. They offer 2 XP every 2 hours.