I feel as though early bug types in general (caterpie, weedle, sewaddle, spinarak, wurmple) are all underappreciated.
Starting with gen 1, Butterfree and Beedrill are both capable of dealing plenty of damage, Butterfree being more up-front and status effect oriented and Beedrill being more sustained with moves like fury attack and twinneedle.
Spinarak and Ariados, I find, are pretty strong when you teach them poison type moves such as sludge bomb and toxic. Their defense may be lacking, but their speed makes up for it.
Beautifly and Dustox are two amazing bug-type pokemon in that Beautifly can deal heavy special damage and heal itself with morning sun, and Dustox can endure quite a bit, as well as deal out damage.
Wormadam is pretty unique in that its type changes based on where you found one. I think that little bit of diversity allows you to have more than one in your party with different specialties, which can be useful in mono-type runs. Mothim is in the same place as Dustox, except it's less durable for more damage.
All I have to say in defense of Sewaddle is this: Burgh's Leavanny. Remember that guy? He can wreck entire parties if you're not prepared.
I have to go with Zubat.
I used to despise Zubats. Then I finally threw one in my team for the heck of it. Leveled it, evolved it to Crowbat. Crowbat's not a bad pokemon. I'd take the Zubat-Crowbat line over Pidgey-Pidgeot any day of teh week. Good mix of great flying attacks, poisoning/confusing effects, and dark moves. I seriously regret the time I overlooked this very good mix.