I know you said top 5 characters, but i couldn't pick just 5 characters. So here is 9 One Punch Man characters. Saitama, Genos, Tatsumaki Tornado of Terror, Fubuki Blizzard, Speed-O-Sound Sonic, Mumen Rider, Garou, King Engine and Lord Boros.
I like more characters. Just adding a few for now.
Top 5 One Punch Man characters.
#3 Tatsumaki Tornado of Terror
#4 Fubuki Blizzard.
#5 Speed-O-Sound Sonic.