Importantly is an adverb and should not be used except to modify a verb. In a sentence such as "Most importantly, we completed the project on time," the only verb is completed. Because "importantly completed" is nonsensical, it is obvious there is no verb for importantly to modify. Although some authorities do accept either word in such sentences, important is the better choice. Some speakers and writers, even experienced ones, use importantly rather than important, perhaps because they imagine it sounds more important—not importantly.