I'm not familiar with the series so I researched it... in case anybody is interested, the bald one in the forefront is Saitama, the one with the cool looking arm is Genos, the one with the purple eyes on the left is Onsoku no Sonic (or just Sonic), and the old man is Bang (also known as Silver Fang). I looked for name information for the double chin kid but I think he's a nameless character (for now).
According to Genos, it's "Speed of sound (lol) Sonic". But you've still got to say it with a serious face.