What is the best wood to use for sash windows?

Hardwoods are always better, but there is no reason why you can not match the existing timber the window is made of. Readily available and relatively cheaper hardwood timber is Meranti, which you can pick in most timber yards and Dry Flex repair system is one of the best ways to go about it ( although it might be a little cumbersome for non-professional users) . Window repairs that are beyond cosmetic and require removing the sashes from the box frame can be costly and on many occasions where damage to the joints is extensive (due to rot etc.), it is advisable that the sash/casement window is replaced with identical new one. Hope this helps.

Accoya

hard wood

so i restore a lot of windows. mainly on listed properties. I always use the same timber as the existing window. I would expect a window on an older property to be softwood so therefor a joinery grade timber is suitable. sills are sometimes hardwood generally oak, sometimes sapelle. your best bet is to look to a joiner or joinery shop to supply the sections. also someone who has a good knowledge off window restoration to do the work