Easy build:
Rod of Ages -> Deathcap -> AP (I use Boots of Swiftness most of the time with occasional Mercs)
Moderate build:
Catalyst -> Tear -> Whatever You Feel Like That Gives AP
Hard build:
Whatever The Hell You Feel Like (Tear -> Revolver -> Spirit Visage; Tear -> Warmog's -> Mejai's; Banshee's -> Frozen Heart -> Rylai's; whatever the hell you feel like)
The trick is, of course, adapting. Anivia is probably my most varied caster in terms of builds.