Exeggutor is definitely the best, if there is a best.
He is useful in the early-, mid-, and end- game... Eggy can sleep, stun, and explode. Did you read that? Sleep Powder, Stun Spore, and Explosion! So boss.
Exeggutor's 348 special and other fantastic stats make him an adept mixed sweeper and a special sponge. He walls Rhydon/Golem, Gengar, Dugtrio, and Alakazam. He's also one of the few pokemon who is bulky enough to use Rest well (Lapras, Hypno, Zapdos, and Articuno being the others). He can competitively use Leech Seed, Reflect, and Mimic, too.
Let's also remember how easy it is for Eggy to switch in on EQ, which allows for fantastic team synergy with Gengar, Golem/Rhydon, and Counter users. He also has great synergy with Lapras by baiting a Blizzard and having the foe waste precious PP, and with Chansey by being able to switch in on Psychic when special falls start to add up.
Exeggutor provides so much dynamics to the team, it's incredible. Start paying attention to how many times you switch in Exeggutor (or threaten to switch him in) during your battles, and then you'll give him the appreciation and respect he deserves.