Use the adjective unforgettable to describe things that are so meaningful or important that you could never forget them, like the taste of your grandma's apple pie or your first kiss.
Things that linger forever in your memory are unforgettable. You might describe a devastating house fire this way, or your first trip to Paris. While the meaning of unforgettable is easy to decipher once you spot the root word forget, it's a lot harder to spell. Remember the double t and the able ending, and you'll do fine.