glänzend
shiny
Describing a surface that strongly reflects light and shines. The opposite of matte.
Sie polierte das Silberbesteck, bis es glänzend war.
She polished the silver cutlery until it was shiny.

shiny / excellent
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shiny
Describing a surface that strongly reflects light and shines. The opposite of matte.
Sie polierte das Silberbesteck, bis es glänzend war.
She polished the silver cutlery until it was shiny.

'Glänzend' is formally the present participle of the verb 'glänzen' (to shine). Like many participles, it can be used as an adjective to describe a quality that is currently active—something that shines.
Die Sonne scheint auf die glänzende Wasseroberfläche.
The sun is shining on the shiny surface of the water.
The meaning of 'glänzend' depends on the noun. If it refers to a surface (paint, eyes), it means 'shiny' or 'gleaming'. If it refers to something abstract (idea, result), it means 'excellent'.
Das Auto hat einen neuen, glänzenden Lack.
The car has new, shiny paint.
In the figurative meaning 'hervorragend' (excellent), 'glänzend' is often used for successes and positive prospects. Typical phrases are 'glänzende Noten' (excellent grades), 'eine glänzende Karriere' (a brilliant career), 'ein glänzendes Geschäft' (an excellent deal/business), or 'eine glänzende Zukunft' (a bright future).
Mit diesen Qualifikationen hat er glänzende Aussichten auf dem Arbeitsmarkt.
With these qualifications, he has excellent prospects on the job market.
When 'glänzend' describes a person, it often refers to their appearance or mood. The phrase 'glänzend aussehen' means that someone looks very good, healthy, and rested.
Du siehst heute aber glänzend aus! Warst du im Urlaub?
You look absolutely radiant today! Were you on vacation?
successful
'erfolgreich' and 'glänzend' can both describe a very good result. 'erfolgreich' is neutral and says that a goal was reached. It fits exams, projects, conversations, or companies. 'glänzend' is stronger and more praising. In its figurative meaning, it means excellent or especially good. It often sounds a bit more enthusiastic than 'erfolgreich'. If you only want to say that something worked out, 'erfolgreich' is more natural. If you want to evaluate the result especially positively, 'glänzend' can fit well.

Ihre Präsentation ist glänzend gelaufen, und alle waren beeindruckt.
Her presentation went excellently, and everyone was impressed.
Wir haben das Projekt erfolgreich abgeschlossen.
We completed the project successfully.