Is there a grammatically correct replacement for "a whole nother level" that retains the intended meaning and emphasis?

"Another" is ambiguous: "another N" can mean "an additional N" or "an alternative N". So, according to which of those senses you mean, I suggest "a whole other level" and "a whole different level".

Instead of "a whole 'nother level," you could say the following: paramount, superior, revolutionary, unparalleled, or incomparable.

Of the words listed above, I think "unparalleled" best conveys the intended meaning of your sentence.

"I had had great experiences before, but this one was unparalleled."

If you want, you can qualify unparalleled to be more specific: unparalleled in its _______ .

"Whole 'nother" is often paired with "use ta could". Those who use these types of colloquialisms often have additional meaning than a more grammatically correct term. Any word/phrase you can use to get another human to understand you is good, spelling it with correct punctuation can indicate that is the way you intended, if not using the latin (sic). Dadda beda!