Rhydon or Exeggutor

If you want high placement --> Rhydon.

If you want a really good defender --> Eggs.

High placement tends to prioritise as a gym defender so long as there is a blissey or snorlax already in the gym

I'd go with the TreeHead as long as he isn't going to sit on the bottom of any gym you place him in. The double weakness to water and grass is way easier to exploit than a weakness to bug. Just a few missed dodges and that bug is much easier squashed than a Vaporeon.

I would wreck either of them, regardless of their level or my attackers level. That being said, I find rhydons moves easy to dodge, and being double weak to vaporeon is a deal breaker for me. Missing dodges does not deal that much more damage.

I use a Bite Flamethrower Arcanine to destroy any exeggcutor. Confusion is a devastating fast move if you miss dodge. It takes away a good 10-15% of my Arcanine's HP if I miss the dodge. It almost always get in one Solar Beam too, and I beat them in around 30-35 seconds.

If it will not be in the bottom the gym, I always like putting my exeggcutors in gyms. I powered up three confusion psychic and seed bombs before the moves changed. I have two high level Conf/Solars with 89% IVs that I should power up.