Nominate tangrowth for c rank, maybe even b...
Disclaimer: i have no experience of using tangrowth, but have battled against it on many occasions and personally find it more annoying than amoonguss.
Tangrowth plays similarly to amoonguss, it also has rage powder/regenerator, a sleep move, and is quite slow. It distinguishes itself by having greater physical bulk, a ground-type resist (in particular, it takes earthquakes like a boss) and more attacking-wise, with a power whip and/or leaf storm than can do serious damage. On the other hand, it has less special defense, is weak to bug and lacks the fighting-type resist, and sleep powder can miss at the worst times. It could try its hand at a chlorophyll set, but there are better sun sweepers out there.