der Knaller
hit
Colloquial for something that is a sensation, a great success, or especially impressive.
Das neue Album der Band ist der absolute Knaller!
The band's new album is an absolute hit!

hit / firecracker / bombshell
Loading your saved state.
hit
Colloquial for something that is a sensation, a great success, or especially impressive.
Das neue Album der Band ist der absolute Knaller!
The band's new album is an absolute hit!

'Der Knaller' (the firecracker/hit) is derived from the verb 'knallen' (to bang), which means 'to make a loud bang.' The literal meaning is therefore a firework. The colloquial meanings describe something that has a similarly strong, sudden effect.
An Silvester hört man überall die Böller knallen.
On New Year's Eve, you hear firecrackers banging everywhere.
Most meanings of this word are colloquial. When 'Knaller' means a sensation, a surprising piece of news, or an attractive person, it is used only in informal conversations. Only the meaning 'Feuerwerkskörper' (firework/firecracker) is neutral.
Die Party gestern war echt der Knaller!
The party yesterday was a real hit!
You can recognize the meaning from the context. If it is about 'Silvester' (New Year's Eve) or 'Feuerwerk' (fireworks), it means the firecracker. With 'Film' (film), 'Lied' (song), or 'Party' (party), it means a success. In connection with 'Nachricht' (news), it means a surprise.
Hast du schon gehört? Sie ziehen nach Australien! Das ist ja ein Knaller!
Have you heard? They're moving to Australia! That's quite a bombshell!
In the meaning 'Sensation' (sensation/hit), 'Knaller' is often used as a predicative noun with the verb 'sein' (to be). It is frequently intensified by adjectives like 'absolut' (absolute), 'echt' (real), or 'total' (total).
Dieses Computerspiel ist der totale Knaller, du musst es ausprobieren.
This computer game is a total hit; you have to try it.
surprise
'die Überraschung' and 'der Knaller' can both mean an unexpected event or a special piece of news. 'Überraschung' is neutral and fits almost any situation, including small private things. 'der Knaller' is colloquial and much stronger. You use it when something is especially impressive, very successful, or also shockingly unexpected. In formal texts, 'Überraschung' sounds more natural. Among friends, 'der Knaller' can sound very lively.

Das letzte Lied beim Konzert war ein echter Knaller.
The last song at the concert was a real hit.
Vor dem Termin bekam Paul eine Überraschung: Sein alter Freund kam auch.
Before the appointment, Paul got a surprise: his old friend was coming too.