Which is grammatically correct A once-in-a-lifetime experience or an experience?

Both "a once-in-a-lifetime experience" and "an experience" are grammatically correct. However, they have different meanings and are used in different contexts.

* A once-in-a-lifetime experience refers to a very special and unique experience that is likely to happen only once in a person's life.

* An experience is a general term that refers to any event or activity that someone has lived through.

Here are some examples:

* "Visiting the Taj Mahal was a once-in-a-lifetime experience." (This implies that the experience was very special and unique.)

* "I had an experience at the grocery store yesterday." (This is a general statement about an event that happened.)

The choice of which phrase to use depends on the specific context and the intended meaning.