
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
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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!

firecracker
A firework that mainly serves to produce a loud bang; a banger.
Die Kinder hatten Angst vor den lauten Knallern an Silvester.
The children were afraid of the loud firecrackers on New Year's Eve.

bombshell
Colloquial for surprising, often shocking news or an unexpected event.
Die Nachricht von seinem plötzlichen Rücktritt war ein echter Knaller.
The news of his sudden resignation was a real bombshell.

stunner
Colloquial for a very attractive or exciting person.
Seine neue Freundin soll ja ein echter Knaller sein.
His new girlfriend is supposed to be a real stunner.
'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.
hammer
Both words can colloquially describe something as very impressive or successful. „Der Hammer“ is the broader evaluation and can also express amazement or outrage, while „der Knaller“ sounds more like a sensation or a major success. Choose according to whether you want a general or more emphatic effect.

Der neue Song ist der Knaller und wird überall gespielt.
The new song is a huge hit and is being played everywhere.
Der neue Song ist der Hammer und gefällt uns allen.
The new song is amazing, and we all love it.