der Renner
hit
A product, song, film, or event that is extraordinarily popular and very successful.
Das neue Smartphone ist schon jetzt der absolute Renner.
The new smartphone is already an absolute hit.

hit / racehorse
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hit
A product, song, film, or event that is extraordinarily popular and very successful.
Das neue Smartphone ist schon jetzt der absolute Renner.
The new smartphone is already an absolute hit.

The word 'Renner' (hit / racehorse) is derived from the verb 'rennen' (to run). A racehorse is an animal that runs fast. A product that is 'der Renner' (a hit) is, figuratively speaking, 'running' because it sells very quickly.
Dieses Spiel ist bei Kindern gerade der absolute Renner.
This game is an absolute hit with children right now.
The context makes the meaning clear. If people are talking about products, films, or music, it means a bestseller or hit. If the topic is horses, breeding, or horse racing, it means a racehorse.
Der neue Krimi der Autorin ist ein echter Renner.
The author's new crime novel is a real hit.
In the meaning 'bestseller' or 'hit', 'der Renner' is a colloquial expression. It is often used in spoken language and informal texts.
Die Lederjacken waren in den 80ern ein echter Renner.
Leather jackets were a real hit in the 80s.
'Renner' is often intensified with adjectives such as 'absolut' (absolute), 'echt' (real), or 'groß' (big). Another typical expression is 'zum Renner werden' (to become a hit).
Das Lied wurde über den Sommer zum großen Renner.
The song became a big hit over the summer.
success
'der Erfolg' and 'der Renner' can both say that something is well received or has a good result. However, 'Erfolg' is more general: you use it for an achieved goal, successful work, or also a successful product. 'Renner' is more suited to things that are very popular and quickly excite many people, for example a song, a film, or a product. Learners may confuse the words because a 'Renner' is always a kind of success. But not every 'Erfolg' is a 'Renner': passing an exam is an 'Erfolg', but not a 'Renner'.

Der neue Kuchen ist in der Bäckerei ein echter Renner und ist jeden Mittag ausverkauft.
The new cake is a real hit at the bakery and is sold out every lunchtime.
Nach vielen Wochen Übung war die bestandene Prüfung ein großer Erfolg für Mehmet.
After many weeks of practice, passing the exam was a great success for Mehmet.