My favorite for a LT hold (not a quick trade) is Watts Water Technologies (WTS). It is a small-cap ($1 billion market cap) and is one of the few pure plays in the sector. Over the past several years, they have divested themselves of all other extraneous businesses to focus solely on their water-related businesses.
At present, WTS is attractively valued. Mario Gabelli is the largest institutional shareholder and he is a reasonably astute value investor (excluding his love affair with media stocks).