According to the DB's harvest page, there are 5 Pokémon that have it as a hidden ability.
So there really isn't much choice if you go down the "no skill swapping route".
Pokemon like Chansey and Blissey however, could benefit from Harvest Start Berry for the chance of extra Defense. To do this, however, you will need to take the following precautions,
1. Send out both Exeggutor and your chosen Pokémon.
2. Skill swap so that the chosen Pokémon has harvest and the other doesn't.
3. Switch the useless Pokémon that isn't a Harvest out for Groundon.
Although this method works, its a pain to use around so I'll be giving sets for both Egg and Blissey
Exeggutor @ Starf Berry
EV's: 252 Spa | 252 HP | 4 Spdef
- Sleep powder
- Reflect/Light Screen
- Giga drain/Toxic
This set uses Sleep as a set-up, Sleep Powder the apposing Pokémon to allow Reflect or Light Screen to STP those bugs. Synthesis allows more healing as with Primal Groundon, ( I think). Toxic is for Grass types and Giga Drain is handy.
Now, Blisseys set
Blissey @ Starf Berry
Ability: Harvest ( After the SS manoeuvre above )
EVs: 252 HP | 252 Spde | 4 Defense
- Stealth Rock
- Seismic Toss
A brilliant set for Blissey, the move Seismic Toss will do exactly 100 or 50 depending on your settings. T-wave, Toxic and Stealth Rock help that little bit out and make this Blissey more formidable. Softboiled is for recovery.
Whew, that was long. I hope these facts help you make an informative decision about who to use.