Best insulation for a converted loft space?

Rigid foam boards or mineral wool batts are solid choices - foam gives you better R-value per inch if space is tight, while mineral wool is easier to install around awkward angles and pipes. if you're doing this yourself, mineral wool tends to be more forgiving since it doesn't require a vapor barrier in most climates. Just make sure you've got proper ventilation up there or you'll end up with condensation issues that'll wreck everything.