Unlike most tanks, Alistar doesn't need a whole lot in terms of defensive items to be an effective tank. His natural HP and ulti are about all he needs to belong on the front lines. The problem with Ali is that his defensive nature makes him unthreatening. Proper itemization is necessary to allow Ali to become a major obstical standing between enemy dps and your squishier teammates.

Play guard dog for the dps/supports. Who ever they focus, you use an ability to peel them and bring down the pain on them as well. All the while, spam your heal to mitigate harass. Do this until one over commits, if one never does you can push them into their tower and whittle the tower down.

If possible you can headbutt carries or supports to locations where they can be focused as well.

Alistar is also a great initiator and chaser.
Protip: Headbutt and Pulverize can be chained to cause an effect similar to Malphite's ultimate. When in range for headbutt, use the ability imediately followed by pulverize. Alistar will lunge forward and pop the enemy up in the air instead of back and deal both skills damage at once. This is a great way to initiate against clumped enemy teams (as pulverize is aoe and will hit all enemies in aoe range) it is also a great way to stop a fleeing opponent from escaping. It also does an incredible amount of burst damage once you have a decent amount of AP- making it excellent for harassing, or bursting down opponents. It is especially effective against extremely squishy but mobile opponents such as Eve and against characters such as Trynd which rely on activating CDs to survive focus.

CDR is everything to Ali, it is survivability (Ulti/heal), it is offensive capability (awesome ap ratios on abilities), and it is defensive capability (stuns, push backs).

HP-Ali's ult will not protect you from true damage abilities. This means Olaf, Cho, and ignites are going to tear you apart unless you have stacked hp to some extent.

HP stacking is often dangerous as it allows % based abilities and effects to hit very hard. Alistar however has such incredible mitigation in his ult and such amazing CC that this really isn't an issue. If a madred's user focuses you, if he is a ranged you can get to him and simply kill him or LoS him because of your huge mobility. If he is melee you can easily kite him due to your mobility and the scepter/passive combo.

Flash: Flash is a must have for any Alistar. You can flash in and pulverize then aim up a headbutt to punt them to desired location, or you can flash behind them and headbutt them up you lane and pulverize them to stun them in a tower or in an allied gank pack. It of course also is great for escaping when needed (which is necessary occasionally due to most of your survivability being in your ult duration.)

Exhaust: I know that there are several ghost Alistar fanboys out there. Let me put some logic to why you ALWAYS choose exhaust over ghost.

Alistar's goal in life is battlefield control. You protect squishies, and you help focus targets get focused. Being able to ghost and chase after someone pales in comparison to slowing them down so your teammates can kill it in the first place. Also, with the changes to the skill, it is now a very powerful tool to protect squishies against ALL carries (AP included). Simply put, one exhaust wins team fights where ghost will not.

Masteries:

The main idea of this build is CDR and as such the utility is very promising. Extra mana and mana regen will help since you will use abilities more often and burn mana more quickly. Extra movement speed will help since Ali is inherently slow. Improving summoner skills will help both offensively and defensively. Again we want this Ali to be feared and as such getting to AK in the offense tree is highly beneficial.

Alistar is one of the most survivable tanks in the game, hence it is ok not to have defensive masteries.

Skillz:

Your first skill to level is Pulverize. This skill will allow you to farm effectively, is an effective stun/disable and scales really well with AP (1:1). Not to mention it is an AoE ability which is extremely effective against clumped melee.
With the addition of flash this skill is a killer. Flash on to an enemy and pulverize immediately for ganks. A single blow to an equally geared player while laning will do 25%+ (40% is common against the squishier champions) and toss them in the air giving time to line up a headbutt. It is recommended that you get a player to 60% before using flash and completely commiting to a gank as to insure a kill.

Second skill to level is Headbutt. This skill will allow Ali to dash to a fight, is very effective at protecting squishies, scales well with AP (1:1) and can be used to punt enemies into your team's range for focus.

Unbreakable Will is of course your ult and should be kept at max available level. Use this ability when being focused, CC'd, and if you have the mana- i.e. mana for atleast 2 spell rotations and the ult- while ganking. It makes you nearly unkillable. It allows you to tank towers on dives early on and push down towers late game. It is important to play defensively while this is on CD as it is your main source of mitigation. It also breaks crowd control effects and significantly increases your auto attack damage (great for pushing towers).

Triumphant Roar is a great ability. It's cheap and on a low cd so even at a low level it can semi-effectively harass heal. But I recommend only taking one level of it until late game simply because it does not fit the flank-tank-spank play style represented here. When you do hit level 18 your option to support heal with this build also will open up. Also at this point Ali will be nearly unkillable even without his ult because of this ability, high AP, and full CDR.

Runes:

This is an initiator tank build. It is designed to pick a target, get to that target and move it to a position where it can be focused. After this point the battle is already won. This being said, Ali has all the survivability he needs in his ult. We want him to play a supporting role doing major damage while keeping the enemy team tied up.

I chose spell pen marks even though this is a hybrid build because it will help tremendously in the early game. It will help with farming and in lane fights as most your early game damage is from pulverize.

Armor seals will help with early game survivability and open the option of jungling earlier and is recommended because little armor is represented in itemization.

Straight CDR glyphs will help get you to the CDR cap and help early game to win lane fights.

Magic pen quints will also help Ali bring the hurt.

start with: mana regen pendent + hp pot+ mp pot
I use this initial load out because it allows you to control the lane and support your partner. The extra mana regen allows you to harass effectively and spam Triumphant Roar.
The two potions will insure that you will be able to maintain harassment and keep yourself alive through enemy early focus.

The boots of swiftness are amazing for the lane phase. Being able to position yourself and gank will put your carry ahead of the curve. Rush these ASAP.

My next purchase is a Rylai's Crystal Scepter. This item is extremely powerful on Alistar (perhaps more powerful on him than any other champion). The slowing affect will trigger on your passive, and will give the full benefit slow on a headbutt target.

This means that all your abilities (including heal and ult) will provide an aoe slow that will last the duration of your passive (4 seconds) +1.5 seconds (slow duration on scepter).
This means you can permanently slow an enemy in addition to the knock up/stun and knock back of pulverize and headbutt. This obnoxious amount of CC makes Ali formidable to say the least.

The item also gives health and ability power. The considerable amount of health on the item will help you tank effectively while the AP will make you hit very hard.

I next purchase a Fiendish Codex. The CDR this item offers makes it a worthwhile investment. At this point in the game you can run through enemy focus while ulting and not worry about being killed- the CDR is important because it will allow you to have your ult up faster for more initiation capability. The extra AP is also amazing. It will help your farm considerably and you can really punish enemy carries because of it.

I now keep working on my hp while stacking armor and magic resist. I tend to favor items with an offensive gleam in addition to defensive stats.

I love the banshee's veil on Alistar. It provides extra mana to keep you going in extended fights, extra hp, and the spell shield is amazing while initiating. Often the enemy will hit you with a cc right off when you charge in. Blocking the first allows you to get closer before they can hit you with a second. Ult the second CC and flash if necessary to position. The Veil allows you to position that much more easily while providing MR, HP and Mana at the same time.

I next buy a Sunfire Cloak. This item is really mean. It provides even more AoE pain to Alistar on top of his passive and pulverize. It also will give bonus HP and some armor.

I then get an Abyssal Scepter. The scepter is incredible as your damage is almost exclusively magic. It will even help your sunfire deal more damage. Not to mention it provides your AP carry the debuff as well, and gives you more AP with that extra MR. A very worthwhile investment barring no one else on the team has one. If someone else beat you to this item consider a Force of Nature instead.

I then upgrade my Codex to an evil tome. The codex will improve your CDR, provide more mp5 and an AP upgrade. Its everything we like about the codex but with a stat upgrade.

Building Alistar with an offensive nature is honestly the best option. This build boasts a great combination of personal and party utility. I've seen several Ali's build for auras, for survivability, or for damage but in the end, all these builds lack key components which make an affective tank. The team mitigation via CC that this build offers is far greater than passive mitigation achieved by auras. The offensive aura Alistar build lacks personal utility leaving a gimped and underfed tank by mid-late game due to lack of farming potential and has very limited survivability. The straight AP Ali likewise lacks survivability and cannot support teammates as well. Defensively stacked Ali builds can be completely ignored. This build, however,has it all.
Though slightly unconventional, this only serves to give you more of an edge due to them not knowing what you are capable of.