Singed

Sapphire Crystal
Catalyst the Protector
Advanced Boots
Rod of Ages
Force of Nature
Rylai's Crystal Scepter
Guardian Angel
Rabadon's Deathcap

(credit to Keels on mobafire.com)

Volibear

Regrowth Pendant
Philosopher's Stone
Advanced Boots
(next is either Spirit Visage or Warmog's Armor, get both)
Atma's Impaler
Rylai's Crystal Scepter


(credit to Badass Prince of LoL on mobafire.com)

Skarner

Regrowth Pendant
Philosopher's Stone
Sheen
Advanced Boots
Stinger
Nashor's Tooth
Phage
Trinity Force
Hextech Gunblade
Warmog's Armor
Ionic Spark

(Credit to ninjabrain on mobafire.com, except the Ionic Spark)

Cho'Gath

Boots (build advanced boots as needed later)
Catalyst of Protect
Glacial Shroud
Advanced Boots
Frozen Heart
Rod Of Ages
Banshee's Veil
Rabadon's Deathcap