ITR and B16 are both the same casting, PR3 is the code on those heads. The difference is in the porting/valves/valvesprings/valve seats/etc. The ITR head is basically a factory ported B16 head. The GSR head is good as it has good quench areas in the combustion chambers. They are all good starting points for a build, it just depends on what you want to do with them. IMO, as far as straight from the factory stock, 1. ITR 2. GSR 3. B16 Any of them can be made better and are all good parts to start out with. Specific applications can also affect the decision due to Compression ratings with different head and block combos.