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It's really dependent on your play style.
1) Aggressive in your face lux
DFG and Lich Bane are great but you also need to build some health. The damage scales with the mana due to Lich Bane and your passive. However, this is very risky and you need to be careful. If you're going up against a gap closing champion, this probably isn't the best build because most gap closing champs have more burst more frequently. Lux has great burst once in a while.
2) Long range rainbows
TONS OF AP and CDR
If you have blue or Athene's, you don't need any other mana regen item.