When you go in the grass coz me and my 'sis getting HG/SS friday.
Rattata, and pidgey are the most common near your hometown your are guranted to get either one of them at least 90% of the time.
Hoothoot, it is everywhere in the tall grass outside of your home town.
These are some Pokemon that you will have to work very hard not to find:
Tentacool (while surfing)