Saitama, of course. He is the main character and his nonchalant attitude towards his overpowering strength is really interesting. He just wants to be a hero for fun and is often bored because he defeats his opponents so easily.
The One Punch web comic can be considered somewhat canonical. It generally adheres to the core concepts and character developments, but there may be artistic liberties taken for visual or narrative purposes.
The Garou arc is really exciting. He starts as a human who wants to be a monster and his battles against various heroes are intense. His growth in power and his complex character make this arc stand out.