A 比 B (更)聪明 = A is smarter than B
A 比 B 还聪明 = A is smarter than B, given that B is already smart
A 比 B 还要聪明 = A is even smarter than B, given that B is already smart ( with some emphasis)
But in real life they are very similar. With or without 要,the meaning of the sentence will not change. I would say 要 emphasizes the fact that A is smarter.