Vileplume

Vileplume is the proud owner of the largest petals in the world.  The larger they are the more toxic pollen they contain.  The large pretals make it hard for Vilemplume to even hold its head up.  When Vileplume walk, they scatter highly allergenic pollen.  They could also shake the petals to emit the dangerous pollen as well.
Vileplume is the 45th entry in the National Pokedex.  It's really slow and has a high Sp. Attack stat.  It makes Vileplume very much of good use.  It doesn't have incredibly high or incredibly low stats other than its Speed.  It's highest stat is just its Sp. Attack.  Its other stats are just- inside the "in the middle" category.  So it isn't really a good Pokemon or a bad Pokemon at all based on stats.  It's just in the middle.
Vileplume has fine Attack and Sp. Attack stats, so you could make it more Physically based, Specially based, or a little bit of both.  It can learn Poisonpowder, which basically just Poisons your opponent.  It's a good move, but Toxic is better, only because it has a higher accuracy.  It can use the classic "Sunny Day-Solarbeam" strategy.  For the daring people, you might want to give it Petal Dance.  It can use Moonlight, which restores your HP, especially when it's in the Sunny weather condition.  Use Giga Drain- (always a useful move).  Basically, Vileplume mainly only has STAB moves.  But most of those attacks are really useful.
Vileplume's ability is Chlorophyll.  Chlorophyll makes your Speed stat go up in the Sunny weather condition.  If you want to guarantee you'll get that Speed boost, use Sunny Day.  This ability is especially useful on Vileplume because it has low Speed stats.
The final verdict is that Vileplume is a useful Pokemon overall.  Not always, but sometimes.  It could be a good Pokemon.  If you want to use one, nobody's stopping you.