Formal:
* Se sentir formidable: This is a very formal and strong way to say "feel fantastic."
* Se sentir en pleine forme: This means "feel in top shape" and is also quite formal.
Informal:
* Se sentir super bien: This is the most common and natural way to say "feel fantastic" in a casual setting.
* Se sentir comme un poisson dans l'eau: This literally translates to "feel like a fish in water" and conveys a sense of being perfectly at ease and comfortable.
* Se sentir au top: This means "feel at the top" and is a slightly less informal way to say "feel fantastic."
Other options:
* Être en pleine forme: This literally translates to "be in top shape" and is a more neutral way to express feeling well.
* Être rayonnant/rayonnante: This means "be radiant" and suggests a feeling of joy and energy.
The best way to choose the right phrase depends on the context and your desired level of formality.