In general, Zubat and Golbat. In the Cerulean Cave, though, definitely Wobbufett. Depending on what happens, this one actually has the capability of KOing your pokemon AND you can't run from it...I have grown to hate it so much.
Freaking PIdgeys. You can be ANYWHERE in the grass..and there it is, just..staring at you. I swear, I have this one pidgey stalking me in the grass just before you enter pewter city. It alwas seems to be a male level three pidgey.
Rattata
Pidgey
Zubat + Golbat
Zigzagoon + Linoone
Diglett
Wobufett
If in the water then wingull,tentacool or if fishing magikarp or carvanah or wailmer.