Well said, Alter. Anyway, my choices would be Tangela and Bellossom. They're great pure Grass types not hampered by Ground moves (as Grass/Poison is all the rage) or mega hybrid weakness. Unfortunately, their lack of decent speed is what keeps them out of the real game (I don't know Tangela's but Bellossom or mine can't clear 200 SP at least), and most of the time, Speed for the first attack counts. I think that's why people don't use them as much.
But the Pokemon with the shortest stick of all is Paras/Parasect as it is slow, two double weakness (Fire and Flying), and its attack moves aren't so strong. However, its status effects are so lovely. Plus, a Bug/Grass combo is a good idea, but it needs a Pokemon that can pull it off. Maybe it will get an evo in the newer Generations and compensate. Maybe.
As to Shuckle or Wobbuffet: Shuckle has 600 DF and SP DF easy (if trained to 100), so don't scoff as it has the highest defense in the game (so far). Plus, its hybrid mix make it difficult to take it down so easily. Wobbuffet requires an expert to use or someone with knowledge all the move sets and to be aware of all contingencies. It's definitely a "Thinking Man's Pokemon". Since there are those who can't/won't think and/or have the patience to train it, it's why it's so underused, when it really shouldn't be. It's a shame.