Exeggutor
While Tropius is cool and all, it's sadly forgettable. It's too weak to justify being so rare, and it has nothing but its looks going for it. It doesn't feel very "fleshed out", as in having any pronounced characteristics or behaviour, save for that fruit. It's a flying dinosaur growing bananas like a beard. It sounds awesome, but the concept is poorly explored. Every description of Tropius focuses on its fruit, and that only. It ends up being "just another grass/flying".
Exeggutor, on the other hand, sets the tone with its goofy expressions and carefree walk. It's a walking tree, and apparently psychic too, because hey, why not? It's also strong enough to be meritable in-game. It can hold its own in battle, and take on many roles on a team. Sweeper, sleeper or annoyer, Exeggutor has it. It can also go boom, for some reason (learning Explosion). It also helps that Exeggutor requires evolution of Exeggcute, instead of coming "readily available" like Tropius. The sight of an evolving Pokémon mid-way through your ingame run helps build a sense of progress. It's a definite goal to be completed, as you watch your beloved 'mon become stronger upon evolution.
Also, Exeggutor and Exeggcute are the sole users of Barrage. It's a crappy signature move, but at least it is one. It's an acknowledgement of the Pokémon's uniqueness.