Question

Which level is best for giving leaf stone to Exeggcute in LeafGreen/FireRed?

Comments

I'd say "Whenever you feel like evolving it". If you feel like evolving it when it learns a decent enough move, then go for it. Don't let us tell you when to evolve your Exeggcute into Exeggutor. You evolve him/her when you think the time is right.

Answer 1

The last good move it learns by level up is sleep powder, which it learns at level 37. Therefore, you should evolve it at level 37. I think it should know reflect, leech seed, confusion, and sleep powder when you evolve it.
source

Comments

What about TMs? Aren't they important? I feel they are. I have an Alolan Exeggutor that knows Dragon Hammer, Thief, Infestation, and Energy Ball.

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Both Exeggcute and Exeggcutor had the same TMs during gen III FR/LG, albeit Exeggutor also has hyper beam.

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What about teaching it Pshycic instead of Confusion?

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You can teach it psychic whenever you want, including after it evolves. The four moves (reflect, leech seed, confusion, sleep powder) assume you don't like using TMs.

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Energy Ball does not exist in FR/LG as that was'nt introduced until D/P/P.