To be specific it’s for K-exeggutor with it’s HA and I want to make use of it but I don’t know what berry to give him.
I would say Sitrus Berry. Once you reach half health, it restores HP, which is nice, and Harvest bringing that back is even better.
Most of the Random Battles I've participated in that had an Exeggutor with Harvest used Sitrus Berry, so Smogon also seems to think Sitrus Berry is the best choice.
Hope I helped!