Rod of Ages...
Why is it in no guides, and does no one build it? It seems extremely efficient, and doesn't take all that long to full stack. I realize mana is meh these days unless you get Archangels but 500 health and 100AP on a single item seems super efficient to me. Not counting Eternity so much as it's kinda meh. It seems pretty good on almost any champion that uses AP at all.
Rylais is 200 less health, 10 less AP, with much more utility granted for 100 more gold, but it seems strange people don't consider ROA an option at all.