If the run from the house to the street is short (under 60 feet) I'd use *Type L * soft copper. It is less likely to break and it has no fittings in the ground except at each end of the pipe. I'd also put pipe (foam) insulation) around the copper run. For longer runs my next choice would be schedule 40 PVC pipe. Not a bad choice at all. I would not use flexible plastic - that is black *poly* pipe. It comes in a roll. Way to soft and the metal clamps and hard plastic (or metal) connectors will break over time. I repair them weekly.

My Black Poly Pipe Pet Peeve My experience with black poly pipe (comes in a roll) is that it leaks. It is too soft. Hard surfaces (rocks) rub holes in it and where ever there is a transition to another type of pipe or an elbow it will leak there - because of the hard adapter, radiator clamps and the soft pipe. Schedule 40 PVC or copper is the best way to go with water mains.