Exeggutor with Harvest is common. Exeggutor with Starf Berry is not. The Starf Berry is actually really good. It's essentially a free Swords Dance, Iron Defense, Nasty Plot, Amnesia, or Agility when at low health, and you can get multiple boosts thanks to Harvest. So here's a Starf Berry Harvest Exeggutor:
Exeggutor @ Starf Berry
252 HP / 252 SpD / 4 SpA
-Sleep Powder/Leech Seed
Substitute is near-necessary HP loss to get the Starf Berry boost. Sleep Powder allows for Giga Drain spam, and Leech Seed provides constant HP drain, whichever one you prefer. Infestation is continual damage, which is nice. The Starf Berry boost will almost always be good. Its Defense is pretty low, so +2 Defense is good. +2 Special Attack boosts Giga Drain, +2 Special Defense boosts its even lower Special Defense, and +2 Speed boosts its horrible Speed. +2 Attack is the only bad thing.