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Gens 2, 3, 4, 5, and 6 all have a cluster of really hard-to-place Pokemon. I don't believe any Pokemon is automatically forgettable but in a group of a hundred choices a few will naturally fall into the shadow of obscurity cast by the ones that get the attention.
Some of the ones I always forget are Pineco, Tyrogue, Clamperl and its evolutions, Chingling and Chimecho, Burmy, Finneon, Woobat, Basculin, Ducklett, Spritzee, and Swirlix.