Originally Posted by Daiko
By the time you can reach Tentacool you should be at a level where they aren't too much of a bother in battle and it is just the frequency that they appear which is annoying.. Even then you only find them in Water.
Zubat on the other hand is found in Grass (at night) AND Caves from a fairly low level where you can often struggle to deal with the Confusion/Poison. It also has two evolutions that just take what it already does to a whole 'nother level of annoying!
In my opinion, Zubat wins.. Or loses, I'm not sure which here.
This. Zubat wins.
Take Pokemon Red/Fire Red for example, in Mt Moon near the start of the game you encounter so many Zubat, and they'll constantly confuse you with Supersonic, sap your health with Leech Life and flinch you (in Fire Red) with Astonish. Later on in other caves you'll encounter Golbat's who are just as annoying, and often Rockets have Zubat/Golbat's to annoy you with. You don't really encounter Tentacool's until you've got Surf, and by the time you have Surf you're more than capable of dealing with them, and you'll usually have a electric type to easily beat them.