You must place "handsome" and "great" in quotation marks. When you talk about a word rather than actually use it, it is essential that you place it in quotes. Replace "as great" with "great". "Unlikely" doesn't make sense. What you mean to say is "unlikely to be true". "Said so" is a poor word choice. It is better to say "it is better to let others say it". Your sentence would improve if it used verbs. The only true verbs in your sentence are "seems" and "is". There is nothing wrong with those verbs but they serve no purpose in your sentence other than to bind adjectives together. Your sentence contains no action verbs. This makes the sentence weak even if it contains no grammatical errors. ("Declaring" (noun), "said" (adjective), and "calling" (noun) are not verbs.) Here is a suggestion for illustration. I have put all the action verbs in caps: When you DECLARE yourself "handsome", you CONVINCE no one and perhaps REVEAL yourself as insecure. It is better to LET other people SAY so. The same APPLIES when calling yourself "great".

Declaring (you wrote a typo) Best to be told so by others Calling yourself great (no need to say ‘as’)

What you write has grammar mistakes but more importantly doesn’t have a clear meaning. The statement is about your belief that others shouldn’t comment on themselves, but you don’t explain why you feel that. Instead, you speak as if your belief is some kind of universal truth.