If you're playing an AP Alistar and you find you're not being targeted, build a Nashor's Tooth. It sounds a little awkward, but for the love of all that is holy, this shit works. If you already have tank stats and you're lacking offensive capabilities, the item does work very well. It has great synergy with your durability, since you'll be alive long enough to make use of the passive. The rest of the stats are all useful. However, I would not recommend building this item until you have one or two items, unless you're trying to be a backline diver. In short, Nashor's Tooth gives the sustained damage output that you're lacking compared to the AD build.
I'll comment a little bit on how I play Alistar in competitive. I basically just hug my carry. That's it. You are the GTFO button for your carries. The most important part of this is having cooldowns to match your opponents' cooldowns. Tank stats are necessary, since the longer you live, the longer you can peel. The trick is understanding just how much tank you need for the task you are trying to accomplish. In this case, it's just enough to survive as long as your carries. If your carry is dead, you should be running and not fighting (unless you are the carry but shhh).
