Well, since Exeggutor is your favorite, you need to choose either Tangela or Articuno to replace with something that isn't so vulnerable to Fire, because as it is you have three pokémon weak against it and none who can dependably withstand it. Your replacement shouldn't be weak to Fighting either, because that would form another 3x weakness with Persian and Snorlax (although you at least have Exeggutor to eat Fighting attacks.)