Which are all the S and A class heroes whom Garou fought?
This will include spoilers of necessity.  I'll mention the webcomic appearances for S-class (I think the manga covers all of the A-class he fought in the webcomic already, but I won't quite guarantee it).  Though keep in mind there are differences between the webcomic and manga.

S-class


Metal bat
Genos
Bang
Watchdog Man
Tanktop Master
King (sort of)


In the webcomic he also fights:


   Everyone else except Blast, Metal Knight, and Drive Knight.  In several cases he fights three or more of them at the same time.


A-class


Blue Fire
Heavy Tank Loincloth
Magic Trick Man
Golden Ball
Spring Mustachio
Death Gatling
Smile Man
Stinger
Chain'n'toad


There's also a smattering of B-class heroes and even Mumen Rider, and a host of monsters that were classed as Demon or Dragon level threats.  A list of manga battles Garou has fought can be found here.