I love the D2 that comes with the v3i personally, but I also have the 10x6 which I recommend for parks and what not, and then the 4x6 is the perfect little shooter coil for trashy areas. I also would recommend the 18x15 or 15x12 SEF coils for wide open hunting. Those are the coils I can personally recommend, BUT I have heard really good things about the 12" concentric coil "Super 12", tho I personally have never used one I have heard great things about them and I am also thinking about picking one of them up as well. Be sure which ever coil you get, it should be v-rated. I am not sure if the Super 12 comes vrated or not, but I am pretty sure it may.
HH,
Rich