Y8 is the best head.. more compression like he stated above. Im currently running a D16a6 (crx si) block with a Y8 head and Y8 manifold with some other boltons like headers etc..

I have a 91 Civic Lx with a D15B from ex, with 91 si ecu, distr. & empfi, would like to know what single vtec head i can use to create a mini me. =

Use the Y8 or Z6 compression isnt a BIG difference in these two heads but can make a difference.

If you run the D15b block with stock internals (rods, pistons, etc.) with the Y8 head and Y8 head gasket you should be around 10:6:1 compression with 1493.39 cc Displacement.
If usingthe Y8 double layered gasket your compression would be 11:0:1

If you run the D15b block with stock internals (rods, pistons, etc) with the Z6 head and Y8 head gasket you should be around 10:2:1 compression with 1493.39 cc Displacement.
Y8 double layered gasket you should be around 10:5:1 compression with 1493.39 cc Displacement.

cc/Displacement wont really change unless you upgrade to a bigger block.

I hope this gets you looking at your options and helps you out with your swap. Good luck