Good Pokemon that are worth transferring from B2/W2 to X and Y are any Pokemon not available in X and Y and event Pokemon. There are 81/156 Unova Pokemon in the Kalos Regional Dex, which is close to half. 0 of those are legendary Pokemon. The other half is in Omega Ruby/Alpha Sapphire. So what I would recommend to transfer is
Any event Pokemon
Any shinies, of course
And any Pokemon that are not in this list
Hope I helped!