While yes, Saitama can NOW kill monsters with a single punch, perhaps his greatest moment of badassery so far is in the second chapter, during the flashback of his fight with a crab monster. This is when he was just an ordinary guy with no powers. Said monster is initially wiping the floor with Saitama, until Saitama loops his necktie around one of the monster's eyes, and rips it out.
The "Sparring" two parter with Genos. Genos goes for a massive blast that Saitama appears to dodge, but Genos was expecting that and charges at him landing a rain of punches that tear apart the side of a mountain just as his previously fired blast hits. Only for him to discover that what he tackled wasn't Saitama but an after image.
How does the spar session end? Adhering to the rules not to hold back, Saitama prepares to punch Genos but pulls back and hits him with a forehead flick at the end. The after effect of his attack still manages to destroy the mountain behind the two completely.
Saitama gets another one when after destroying the meteor, two B-class heroes try to ruin his reputation, then attempt to execute him in public. While the result was obvious, his speech on why he's a hero in the first place was just awesome.
Licenseless Rider versus the Sea King, an unpowered C-class hero against an unstoppable monster, knowing he has no chance, but doing it because there's no-one else. Though he doesn't win, he saves Genos' life and buys enough time for Saitama to arrive. He even wins Saitama's respect for putting up a good fight.