The best way to build Alistar is to go high health + defensive AP with sunfire. Get Pros Ring + boots then either Merc or Lucidity. You want merc for HARD CC, snares and stuns, everything else doesn't do much. After that I would rush a Sunfire. Why? First you're not lacking in damage due to the Pros Ring and then 40 damage aura, second your becoming very tanky, you'll usually end up being the last one to die in fights meaning that 40 damage aura will do so **** much damage since everyone ignores you. If they focus you, that health will make all the difference, since your ult will work just that much better, as opposed to all the armor you got.