100% Exeggutor Psychic or Grass?

On my Exeggutors without Confusion, I prefer a double grass for synergy/because I have relatively few strong grass mon. That said, this suggests the ideal set is Psychic/Grass for raw DPS.

I have both maxed out Confusion and Extrasensory double-Psychic Eggies and the performance difference is significant. Also, the difference with Alakazam and Espeon in fighting raids (Machamp and Hitmon-bros) is enormous, and its trade of DPS and bulk is nowhere close to Flying Moltres or Rayquaza. Of course, you need rarecandy for these, but they're damn good mons in general too.

In the grass segment though, it pretty much remains the best option in some cases thanks to the Psychic typing for bulk. If you enjoy soloing Tier 3 bosses, current Starmie is a lot less threatening to Grass Exeggutor than it is to Venusaur or SB Groudon because of the Poison and Ground typing, for example.

That depends on what you have/need. As either a psychic attacker or grass attacker, it has a good mix of bulk and DPS. The safest bet is to go psychic so that you don't risk getting Seed Bomb, which is not a good move.
Ex/Sob is a good set for Machamp solos in clear weather, as weather boosted solar beam hits harder than SE Psychic.