The original gasket that came with the car (original equipment manufacturer).

Also, if you take out your expansion tank and dump all that gunk out, then refill with coolant/water, run the car for awhile, the sludge will keep rising to the top of the expansion tank.

Rinse and repeat until you get almost all of it out ...

That is, if you can't bring it in for immediate service.

Good luck

~Paul