Stay away from the NH-90 (too many problems), but if Pakistan can get Congressional approval, and that doesn't seem like too much of a problem, than the S-70b would be my preferred option (not because I'm an American and would like to see American products being sold, but because its one of the most capable platforms available). India's considering the S-70b, and Pakistan could opt for it as well to maintain parity. It's a bit costly, but offers great performance, avionics, ASW electronics and attack capabilities, plus its size would fit most ships in the Pakistani Navy. These wouldn't be second-hand systems either, they would be fresh off the production line.
The Chinese Z-9 would be another good candidate, and one that is already used by Pakistan, it's just less mature than the S-70b at this point in both air-frames respective lives. The Z-9 is smaller and less capable than the S-70 though (being about half the size), but the small size could allow for more to be carried on any given ship. The Ka-28 is a significantly downgraded option too.
