I agree with MrPedantic: as long as the superlative has been used, it seems appropriate to add 'ever', though some might argue that that is a case of 'overkill'.
You don't have to use "ever" to be grammatical. Without "ever" there's no question that the structure is grammatical. However, to be fully idiomatic, you have to realize that, to the English speaker, the structure "... superlative ... (that) ... (have/has/had) Past Participle ..." fairly begs to be completed somehow -- though not necessarily by "ever". "ever" is but one