I've tried a bunch of multis and synth guts as mains with poly crosses. Also tried gut with multi and synth gut crosses. After lots of time and too much money I realized NOTHING compares to gut/poly for its unique combination of spin and comfort. The challenge with gut poly is control and durability.
Too often a soft poly can cause control problems given gut's inherent power.
And whether its poly crosses cutting through gut mains or going dead and harsh well before the gut snaps gut/poly can be expensive with frequent restringings.
So far the best crosses I've found include CoFocus for comfort, durability, and spin, especially in dense patterns; Focus Hex for spin in most any pattern and size; and 4G for control in open patterns.
There is no one "best" cross and it really depends on frame size, pattern, and stiffness. These three have proven great choices for certain frame types while others have defects too great to be viable imo. It all depends on your frame and personal priorities.