Probably Pinsir with Bug Bite/X-Scissor, Scizor with Fury Cutter/X-Scissor, Heracross with Struggle Bug/Megahorn are the best exeggutor counters, as exeggutor has double weakness to bug type, but surely Charizard would take down an exeggutor real quick too. By the way, your Charizard doesn't have the most optimial moveset, it would be even better if it gets Overheat.
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Best exeggutor counter?
Got this 100 IV Charmander -> Charizard. Both mover make extra damage and double resist to Grass.
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Note that the Charizard moveset is a good ways from ideal. As a general rule, moveset is more important than IV.
That said, AS/FB Charizard is a decent counter to a grass moveset Exeggutor. However, there are quite a few better ones. With the listed moveset, several Dragonites would perform better, particularly DT/H; as would a good Scizor, bite Tyranitars, the best Fire attackers, the best Bug attackers, etc. With a FS/OH Charizard, then it's a much closer. FS/OH Charizard would be on par with a DT/H Dragonite as one of the best counters.
If the Exeggutor has Psychic moves, then Tyranitar works best. With an unknown moveset, FC/XS Scizor, DT/H Dragonite and bite Tyranitar do best.
Exeggutor counter list:
1) Scizor, resist both psychic and grass moves. Bug moves have double advantage versus eggy.
2) Heracross - single resist to grass but has bug moves
3) Houndoom - double resists again, fire/dark single advantage. Debatable for second place, I would place it there myself.
4) Your Charizard.
Other things such as Dragonite, Tyranitar, etc are decent for Eggy but not quite as good.
"Other things such as Dragonite, Tyranitar, etc are decent for Eggy but not quite as good"
It's not just a matter of who has the best typing. You also need to consider the strength of base stats and moveset. If base stats + moveset of one species is >25% better than another, it is generally enough to surpass a lack of typing advantage.
DT/H Dragonite has a SE charge move and double resists grass attacks. It doesn't have as good typing as some you listed, but makes up for it with legendary level base stats. So in simulation, DT/H is better against a grass moveset Exeggutor than all on your list except FC/XS Scizor, depending on dodge strategy, relative levels, and how you prioritize health vs speed.
Similarly bite Tyranitar has SE attacks and resists psychic attacks. It doesn't have as good typing as some you listed, but makes up for it with legendary level base stats. So in simulation against a psychic moveset Exeguttor, it outperforms all of above (unless you place strong priority on speed over health).
