Usually depends on enemy team comp for best choice.
High magic: get FoN or BVeil
High Physical, low CC: Go with Last Whisper and more life steal.
High Physical, high CC: More health and health regen. It doesn't help against thornmail but you'll survive longer.
Mixed Physical and Magical team, low CC: Revive armor and life steal
Mixed Physical and Magical team, high CC: BVeil and Revive armor. two items to give you defense, mitigate the magic damage from thornmail, and help you survive.
As always try and pick up an item that helps your dps but also gives you some added health so you can take more abuse while upping your dps.