Lol, all that feedback and I forgot to upvote you ;). I do play Alistar quite differently than you, but I think your guide gives a lot of positive options nonetheless. Thanks for the credit, I'm just getting started here on MobaFire and it's really encouraging to hear that people appreciate others' ideas. Go headbutt some noobs!

This is a great build guide, and has obviously been tested and found worth using. I've fought alongside the author when he used this build, and he was devastating to the enemy. While I feel that Alistar is the most OP tank, that is just the way it always seems when someone is so good with a champion that they are literally unstoppable. I'll be trying out this build myself tonight.
Thanks for taking the time to put this together for those who're willing to try something new, rather than sticking with their previously favored builds.

Been looking for a good ali build. This is the one!

@PsiGuard Thanks for the suggestions. I'll try the other options in place of Thornmail and the Mercury Treads and see what I like. As for the skill changes, I don't use Headbutt that much, I am there mostly to protect the other guys, so doing massive damage isn't my main goal, whereas using the Roar to heal them makes a big difference.

@Canoas, that's a completely different build than mine, and I like the health and mana regeneration of the Pilosopher's Stone over the extra health of the Heart.

Both of you suggest a change to the Masteries, and I'll try the Tenacity out, see if I like it. Thanks for the suggestions!

I suggest you start with boots, get an heart of gold before the philostone and then you get sheen. After that you should get the CDR boots followed by trinity force.
The end of the build is situational.

Flash is the best spell for Alistar because it lets you do an awesome combo. You should also take CV if your team does not have it yet.

Tenacity is one of the best masteries and there it should be taken instead of greed or insight.

Oh I forgot one thing. PLEASE OH PLEASE take off greed (or improved clarity, if you take my advice on the summoner spells) and get TENACITY at the bottom of the defensive tree!!!! ~300-600 gold by the end of the game is NEVER better than 4% damage reduction. Plus that mastery combines with your ultimate to net you INSANE damage reduction when you activate Unbreakable Will.

You have a lot of very good items in your build, but I think I should point out that thornmail is an extremely situational item. It hard counters squishy DPS carries like Ashe or Gangplank, but is pretty much garbage against any casters or tanky DPS. Other items like Force of Nature or Randuin's Omen are safer items to have in your core; thornmail is simply too dependant on the enemy team to get every game.

I play Alistar as one of my main champions. He is among the best crowd controllers in the game and can be very survivable with few items. I can tell you from experience that you should NOT need clarity unless your lanemate needs it. I won't down vote you for liking it, but if you can conserve your mana or use mana/5 runes, it's much more worth it to take flash. Flash, while it is only a "short hop," can save your life, save your teammates, and net you A LOT of kills. Most people know how to stay out of Pulverise range, but most wont expect such a fast assault.

On the subject of Pulverise, it is the worst scaling ability Alistar has. While it can hit multiple targets, both its damage and AP scaling are inferior to Headbutt. I recommend maxing Headbutt first, using it for damage and Pulverise for the CC component. I often try to avoid putting points in my heal spell if I can help it, as it increases the mana cost, which means less procs from your Trample passive.

Other than those few things, it's a very good build and will serve most people well against the majority of team compositions. My parting suggestion would be to add Boots of Mobility as a possible item. It is by far my favorite boots on Alistar, as I find I don't need much tenacity due to my tankiness and cleanse-like ultimate, and the potential for roaming is reason to take Mobility over Swiftness. Another common choice is Ionian Boots of Lucidity, but I find the increased mobility more important than early CDR.