As is usually the case, the "best deal" and "least hassle" are probably different. For least hassle, I'd look at UA, as they have nonstop flights from IAD to FCO. Finding a good deal is harder during the summer, but if price is the biggest concern, you can look at flying to a cheaper city (London, Dublin) and taking a LCC from there.

Other items to consider - FF miles (obviously), and whether you prefer connecting in Europe or the US.