UHMW and HDPE is pretty much like cutting butter. A good quick source for these is a plastic cutting board from your local Walmart or Dollar Store. MDF is pretty forgiving also, and cheap. Keep your feed speed up so the bit is cutting rather than rubbing, the bit will last a lot longer.
I use a lot of solid carbide, 1/4 inch upcut bits from the local home supply store. Good for wood, plastic, and metals.