Viton (FKM) and buna-N both seal well in my experience. If the gasket isn’t sealing well under vacuum or pressure, there’s usually something wrong with the gasket (maybe it’s just a tiny bit ‘undersized’, all too often on cheaper 10" and 12" in my experience- some seem to need a little 'stretch" to sit nicely in the groove)
Make sure both mating surfaces and the gasket are clean, and lube it. Nothing helps a tri-clamp gasket slip perfectly into place as you pull it into place using vacuum, like a little bit of lubricant to let it slide.