Since you already have a Exeggutor AND a Jolteon on your starting 5 I think that the priority for another Exeggutor is not that high (you already have a second one powered up). After that the deciosion for me comes down to DT/HB Dragonite vs. C/DP Machamp. Machamp is more time efficient when attacking Blissey, Snorlax and Tyranitar, but looses Health pretty fast. Still...you won't time out versus Blisseys if you start with him and are able to dodge DG. The Dragonite can kill blisseys too of course, and if your gyms are full of Vaporeon he is probably the better Choice to go first. You will have to max out all three regardless in the end.
My team normally consists of 2 Dragonites with DT/OR, 2 C/SoB Exeggutors, 1 Counter DP Machamp and then depending on the composition of the gym I slot in a DB/DP Dragonite, WG/HP Vaporeon, TS/T Jolteon, FB/B Lapras, B/HP Gyarados or FS/OH Flareon. Still missing a TRex with B/SE or B/C, but will evolve my second one soon after getting IT/FB on the first...
