Mages are the primary users of mana, practically being useless without their spells. Tanks and carries rely much less on it but always find it useful to have their skills available. Mana is a statistic not always kept in mind when making a build unless the champion relies on its intensive use. That is the case of Anivia abusing Glacial Storm or Karthus having Defile always on.
Whether you are building Kog’Maw as a Living Artillery mortar or a Kassadin that teleports instead of walking you will need lots of mana. The following table shows the items that provide mana and their cost effectiveness: the cost of each unit of mana. Let’s click on the last header and see what the sorted table reveals.