If some one should manage to convince me that Suomi M-31 (without wich's assisstance on certain past occasions we'd be speaking russia in here) isn't still the king of the SMGs
The alternative would be MP-5 in 10mm.
If the cost is an object as it often appears to be when police investments are discussed POF (Pakistan Ordnance Factory) made MP-5 clones would be quite promising. The finish is rough and so on but the accuracy is acceptable and reliability is good.
I know one that had been shot ~850 rounds without cleaning without single malfunction.