"Unforgettable" was Nat King Cole's signature song. It's been covered by many people, but his version is the one that will always be remembered. The song itself was written by Irving Gordon, who eventually won a Grammy when the song was re-recorded as a duet by Natalie Cole. Here is a taste of the lyrics:

Like a song of love that clings to me,
How the thought of you does things to me.
Never before
Has someone been more . . .

Unforgettable
In every way,
And forever more
That's how you'll stay.

Here are videos of Nat "King" Cole singing the song and the Grammy-winning Natalie Cole duet. Word is, the song will also be featured in a commercial within the Watchmen movie.