Sheen, Mobility boots, and whatever tank items are appropriate (Randiun's, Banshee's, etc). Sheen helps his early game so he can be a dominant duo partner. The less farmed your opponents get early game the easier the team fights will be later. Plus he is capable of getting kills in lane, especially with a duo partner that has decent burst.

Flash is a must on him, to be able to more reliably pull off his knock up -> headbutt back into your team combo. Make good use of the bushes and force your opponents to get denied or take a lot of damage.

Also remember that heal procs sheen as well....great for last hitting and pushing, since his heal will heal allied minions.

So Im loving the CDR boots. Sheen+Boots are becoming my new core. Makes for a very flexible build for our already non item dependent buddy.

You get ignored regardless of whether u have sheen or not, and late game getting an extra 100 damage on 1 attack is not going to be a difference maker when everyone has their survivability items.

As I stated above try chalice and mp/5 runes. I max Headbutt first and spam it to harass the squishy in my lane until im ready to ghost or flash in to feed him to my lane partner. At worst you force them back or make them blow mana, summoner spells, or healing hurting their laning phase in general.

Trust me I used to run sheen but ever since they enforced the CD on it, it has become a poor choice to get. You used to be able to get three sheen procced hits in between your combo, now that's impossible.

The only time I grab sheen now is if I run a tower wrecking build with him.

His W deals 300 base dmg on lvl 5 with 1:1 ratio. It's awesome.
You also want to be faster, so movement speed quints, boots of swiftness + lichbane. Sheen is also a part of lichbane. After that you can build Banshees/Randuins or just Soulshroud + new offensive Zhonjas.
Depends if you wanna be an off-tank who deals dmg or the only tank on your team.

Never go full tank on Ali. Try to get items that give HP and some side effect like mana,mana regen, or AP. RoA is probably the best way to describe this. His ult already makes you an amazing tank so you need a reason to be attacked. Oh and to further fortify the earlier posts. Sheen is amazing and almost required on him just so he can last hit effectively.

He can be built a variety of ways...though IMO AD>AP. AP has good burst but so does AD really...with his ult on, sheen/trinity procs hit HARD. My beef with AP is that his spells really aren't spammable enough and only 2 direct damage spells. His ult currently bumps his AD. Though with AP his heals are nice, so he's possibly a better pusher since his heals can keep minion waves alive even better.

I think regardless he needs to be built at least half tanky if not mostly tanky. He needs to be up in the middle of everyone's business and can't do that without tanky items.