@batikan - Ultimately what you want are as many the following qualities as possible:
1) slickest surface that is also the longest lasting, which will give the most and longest-lasting snapback
2) lowest surface hardness - to exert as little compressive force on the mains as possible, minimizing denting/notching
3) roundest/flattest contact surface - to spread the string-to-string contact over the smoothest and widest area possible, minimizing the sawing effect
4) if it's a round string, a gage that's roughly within +/- .03mm of the mains, to equalize the applied sheer force on both strings as much as possible
For conventional round strings, probably the best example that satisfies points 1 and 2 would be
IsoSpeed Cream. It's a rubber based partial-poly with a liquid wax coating (that IsoSpeed gives many of their polys) and it's so soft that not only does it tend to hardly notch (or never notch) many non-poly mains (all the but the softest ones, anyways), instead it will actually compress ("pancake") and snap before the mains, kind of like how Silly Putty stretches thin and then snaps. It's a
great cross in this role (for example, I've strung Tecnifibre Triax/RPX/HDMX mains with Cream crosses and have had 4.0-ish clients routinely go north of 50 playable hours with it -- insane combo). A close second would be
MSV Swift, which to my hands is a little bit more firm, but similarly slick.
Beyond that, if you do move to natural gut mains and really want to boost longevity in a hybrid, you definitely want to look into
rounded rectangle and/or oval shaped co-polys, which are also slick and soft. The long side tends to lay flat against the gut, really minimizing the denting/notching, often boosting longevity by 100% or more. The most easy to come by in the US market is probably
Grapplesnake Irukandji (named after the jellyfish....lol). For something oval, there's
IsoSpeed Rexxxer, currently only retailing in Europe and certain other non-US markets, and
StringLab OrbiTour for the Italian/Europe market.
You're bound to get a whole bunch of suggestions of more conventional polys, but ultimately, I think you'll want to stick to the likes of the above for best results.
Hope that helps. Any questions, feel free.