Here are a few options, depending on what you want to emphasize:
Focusing on the sound:
* "Wow!" This is the most straightforward and well-known.
* "Aha!" This emphasizes sudden understanding.
Focusing on the feeling:
* "Oh!" This is short and simple, expressing a range of surprise.
More creative options:
* "Rats!" This could work if the surprise is negative.
* "Madam, I'm Adam." This is a classic palindrome, but might be a bit too long and formal for a typical exclamation.
Ultimately, the best palindrome for an exclamation of great surprise depends on the context and the feeling you want to convey.