I know this fight already happened but it was interrupted because Grimmjow had to hurry. Plus none of them used there released forms. I think if they both fought all out Grimmjow would win. While they are both strong Grimmjow seems to be really durable surviving a lot of serious injuries. Grimmjow wins.

Update 8 7 2020. Even though Grimmjow returned in the final arc he didn’t really get to show any particularly impressive feats that would put him above Ulquiorra. Ulquiorra wins.