Try Shurelya's Reverie, Frozen Heart and Mercury Treads as core items. This will give you 35% CDR, about 2600 HP and 175 armor, 55 magic resistance and 35 tenacity. You also get a 3 second turbo move speed boost from Shurelya's, an Attack Speed debuff for nearby enemies and all the mana you will need.
For the remaining item(s), depending on enemy team comp add:
- For AP heavy teams - Force of Nature or Banshee's Veil
- For AD heavy teams - Thornmail or Sunfire Cape
- For a balanced AD/AP team - Guardian Angel or Aegis of the Legion
- If you want to do more damage (AP) - Abyssal Scepter or Zhonya's Hourglass.
This isn't the highest HP build, but I find the movement speed, Armor, MR and CDR more important and with Alistar's ULT you get 70% reduced damage anyways.
Not counting runes and masteries, with the 3 core itemes, plus BV and Sunfire give you 3400 HP, 220 Armor, 105 MR, 375 movement speed, 35% CDR, 35 tenacity, 1770 mana, a spell shield, a movement speed boost aura, a 20% AS debuff to enemies, and 35 magic damage / second to nearby enemies (an improved "trample" passive). All that for 11800 gold. If you get that far, by end of game, you are pretty much indestructible.