which are the best frequencies on a mikrokit Ap connecting clients using nano stations.

er well. There are NS2's and NS5's. They operate at either 2.4 (NS2) or 5ghz (NS5)

The key with NS integration with Mikrotik is make sure you are using RTS on the Nanostations. Otherwise you will have problems.

Speaking in general terms, if you live in an area that has perfect line of site to your AP go with 5ghz - it is typically less noisy.

If you are in an environment that has light blockage go with 2.4ghz - but use Horizontal Polorization.