cork 7s are easy...they naturally come around...cork 3's are pretty near impossible...to actually cork properly you need to spin more....any cork 3 you see will just be a tilted 3, not truly corked
cork 3,s are really hard, but they can be done. my friend landed one that was really corked when he messed up on a underflip. Corked 180s are also possible, but only for eric pollard and its hard to notice becuase hes not spinning at all.
cork 7s are the easiest as someone said. Cork 5s are a little harder I find becuase its like a cork 7 but I have to focus on stopping myself mid spin pretty much, whereas with a cork 7 i just throw it and it comes around.
I was able to do complety parralel to the ground 360 last year, but now, I cant do them anymore, I think ÉI got some footgae of it, but seriously you dont want to throw a cork 3 if you are not carving into the jump, cause that would suck.
9s are harder than 7s but easier than 5s...i think...all you have to do is throw a little harder than the 7 and keep your body twisting a little more....the first time i tried a cork 7 in the park i overrotated and did a 9 (and beat in really damn hard)