Zubat: - Low Tier
Availability: Available rather early, before the third gym in the Sewers. Golbat can also be caught later on in various caves.
Stats: Super-awesome speed, but it's Attack just doesn't cut it.
Typing: Poison/Flying is actually not as useful as you'd expect it to be, defensively. Fighting and Ground types very frequently have Rock type moves. Offensively, it's quite lousy as well. Poison STAB is obviously one of the worst possible, made redundant by Flying STAB, which itself isn't too great (it'll handle advantageous type match-ups against common trainers decently, but that's about all)
Movepool: Shallow. Good Fly Slave, though.
Major Battles: Absolutely destroys Burgh, and that's it. Marshall will wreck it with Rock type coverage moves.
Additional Comments: I decided to use Crobat because it's one of my absolute favorite Pokemon. But it was totally disappointing. Zubat was a thorn in my side to raise with it's awful stats, while Golbat and, soon after, Crobat were very frequently unable to even 2HKO various things (with an Atk+ nature, no less). When you're relying on Confuse Ray to be of use because your actual attacks aren't cutting it, there's a problem.