Movement Speed Quints
Magic Pen Reds
Armor Per Level Yellows
Magic Resist Per Level Blues
Boots of Swiftness
Rod of Ages
Rylai's Crystal Scepter
Force of Nature
Other Item Choices:
Banshee's Veil (if they are seriously AP or CC heavy)
Frozen Heart (If they are seriously AD heavy)
Moonflair Spellblade (If they are CC heavy and you don't need to tank)
TWO Rods of Ages (if you are getting hella fed)
Singed's biggest problem? So many good item choices make it hard to decide.
Remember, if someone is running away from your team, flash or ghost ahead of them and "lead" them with your poison trail.