PVC is going to be the easiest and will last fine as others said. It makes a great hoop-style greenhouse. One of the problems with it is it can get a little too flimsy if your greenhouse is getting large or when you have snow loads. One solution I saw once (haven't tried it myself) is putting rebar inside the pvc. You can get 20' lengths of both PVC and rebar so you do one continuous piece for each hoop. In the plans I saw (Richo Cech's book on growing herbs), he just pounded some 1" metal conduit into the ground and the 3/4" PVC with rebar slides into the conduit. That makes it easier to take down and put up. He was just draping the plastic down and holding it down with dirt or sand bags. It is meant to be a cheap yet stable greenhouse but is not going to be quite as nice or permanent as one with nice built endcaps.
Also, a conduit bender is awesome. It allows you to build all sort of things from thin-walled metal conduit. I've built a mobile chicken coop and a garden cart that are strong, light, and really durable. I've replaced the wood slats on my garden cart twice and the metal frame looks like I built it yesterday.