Exeggutor advice

Psychic is not subpar, it's just less punishing than it used to be (2 VS 1 bar move). Any Eggy with Confusion is still a beast on defense. Also keep in mind that it gets STAB + Psychic is really tricky to dodge. If you want him as a defender, power him up as you'll not get a better moveset (as Confusion is a legacy move). Even if it's not the optimal charge move, he'll do the job ;-)

The ranking is correct. Confusion hits like truck. This is what you want for a strong defensive exeggutor.

Phychic maybe deals less damage, but it's hard to dodge. So overall a good defensive moveset. Having said that, confusion/seed bomb is superior (the difference is noticeable) and actually I believe is among the best(if not the best) defensive movesets of the game.

But,in the end, I believe if you want a defensive exeggutor, confusion/phychic is very good

If you are a high level player, consider it to power up. It doesn't reach a very high CP, but if you manage to get it a decent spot in a gym he'll do certainly more than good.

Save your stardust for now. If Psychic ever goes back to a two-bar move, then you can feel more confident dumping stardust into it.

Why do you limit yourself only for defense? When you power up certain pokemon, you get to use it for attack too.
Here is another reason to power this up: yes, psychic doesn't have much DPS, but it recover really fast. It is especially useful for attacking Vaporeon with Aqua Tail or even Blissey with Dazzling Gleam. It is amazing for dodging, which is really really crucial. You might be able to fight two in a row with good dodging.

If you have dust and candies to spare I would power it up as a defender. I have 2 maxed out Exeggutors both with Confusion. They hit really hard. Psychic can be tricky to dodge as it comes very quickly.

Timing on Confusion can be tricky to dodge as well, especially the first three. I see no problems powering up that bad boy. Unless you are really hurting for dust.

Go for it. Exeggutor is one of the last non-big 7 mons that can actually stay in gyms and force attackers to carry a specialist in their team of six.

Yes. My 87 iv confusion/psychic lvl30 got a battle won. That's when I knew I had to max him ?

Confusion on D performs better than any charge move setup available. By a lot too.

He is also very serviceable on offense as mentioned above. It's up to you but there is nothing wrong with powering him up.

There's no point in waiting around for a better one, because unless you plan on waiting until they massively rebalance moves again, you're never going to get a better one.

On defense, Confusion>Everything. And Confusion isn't available on Eggy anymore.

Even Psychic isn't that bad; Seed Bomb might be better (though it's more likely to be resisted by the fire and bug types an attacker will most likely be using against you) but Psychic is several orders of magnitude harder to dodge than Solar Beam, which more than makes up for the latter's higher damage. But ultimately, it's all about that Confusion.