Exeggutor ZH/Psychic

it depends on your area and if you have other defenders. In my city gym turnover is quite high and I prefer to power up high CP mons like Rhydon, Snorlax and Gyarados so at least I am not the first to get kicked out.

Zen Headbutt / Psychic is the second best offensive moveset and it is still really good attacking moveset in general. It is also good against those that would otherwise resist grass attacks. Then again, it is the second/third worst moveset for defending Exeggutor.

That being said it can still get high CP and place somewhat high on gyms (after the update not quite as strong as it used to be). It will do alright as a defender, but there will still be a difference compared to Exeggutors with Confusion (not very big). If you don't really have an Exeggutor, I'd say that it can be worth powering up. It can function as both attacker and defender, as all Exeggutor's movesets are relatively close to each other in terms of raw damage output.

It is an okay attacker against a Snorlax with Zen Headbutt, since it resists psychic. Best use for that moveset that I can think of off the top of my head.

Before last nest reset i had an Exeggute nest at nearby park (Magikarps atm), i ended up with 6 good Exeggutors (4 confusion, 2 zen) and boosted them all to level 30. I would use Zen Headbutt only for attacking, it has an advantage over Snorlaxes and Vaporeons (resist) and hits really hard. Psychic is decent attacking move.
Confusion sets are my favourite defenders after Snorlax and Lapras, hits like a truck and quite tanky, and really really strong matchups against the top attackers.
So if you are looking for a defender, try to find one with confusion, you will not be disappointed ^^