sagenhaft
fantastic
Used colloquially to express that something is exceptionally good, large, or impressive; fantastic, incredible.
Der Ausblick vom Berggipfel war einfach sagenhaft.
The view from the mountain peak was simply fantastic.

fantastic / legendary
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fantastic
Used colloquially to express that something is exceptionally good, large, or impressive; fantastic, incredible.
Der Ausblick vom Berggipfel war einfach sagenhaft.
The view from the mountain peak was simply fantastic.

The word is derived from 'die Sage' (an old, legendary story). The ending '-haft' means 'like'. So 'sagenhaft' literally means 'like something from a legend', which explains both meanings: something legendary or something incredible.
Er erzählte von sagenhaften Schätzen, die im Meer versunken sein sollen.
He told of legendary treasures that are said to have sunk into the sea.
You can recognize the meaning from the context. If people are talking about mythical figures, places, or treasures, the literal meaning 'legendary' is intended. If everyday things like food, a view, or a price are being described, the colloquial meaning 'fantastic' is intended.
Die Geschichte handelt von einem sagenhaften Drachen.
The story is about a legendary dragon.
In the meaning 'fantastic' or 'incredible', 'sagenhaft' is a colloquial word. It is used to express enthusiasm and to intensify something very positively.
Sie hat ein sagenhaftes Tor geschossen.
She scored a fantastic goal.
The meaning 'mythical' or 'legendary' is found especially in texts about history, myths, and literature. Typical collocations are 'sagenhafte Kreaturen' (legendary creatures), 'sagenhafte Helden' (legendary heroes), or 'sagenhafte Reichtümer' (legendary riches).
Viele Legenden ranken sich um die sagenhafte Stadt Atlantis.
Many legends surround the legendary city of Atlantis.
incredible
'unglaublich' and 'sagenhaft' can both express that something is very impressive, great, or special. 'unglaublich' is broader: it can be positive or negative and can also simply mean 'very'. 'sagenhaft' usually sounds positive and admiring, for example when talking about a beautiful vacation, a good meal, or a major success. If something is extremely bad or annoying, 'unglaublich' often fits better. 'sagenhaft' is used more when you are enthusiastic.

Der Blick vom Berg ist heute sagenhaft.
The view from the mountain is fantastic today.
Nach dem Training war ich unglaublich müde.
After training, I was incredibly tired.