Alistar is technically better as a support and more conventionally played there. However, if your support is an idiot and picks something like Rammus, might as well get an Alistar on your team somewhere. Also, in top lane his passive is actually useful.

Because all these dumbasses up in top lane be playin stupid **** like Panth and Quinn. Right now you might be sayin "Sensei, why dont we just play Malphite? He can engage like Alistar and is an actual top laner."

To that I will respond using the power of mathematics.
A Malphite ult at 40 percent cdr max ranked has a 60 second cooldown.
An Alistar w q combo with no cdr and each ability at level 1 has a 17 second cooldown.

17 < 60
QED: You're a dumbass

You assert your dominance over the lane. Every time you use an ability, your passive gives you a free sunfire. Use it to pick up cs. Bully the enemy laner by headbutting them when they try to cs. You can counter any engage they do. Take AD runes to pick up cs.

Roam. Kill. Win.

Group. Kill. Win.

Alistar is versatile. Once Zzzzzzzzzzzzz'rot comes back build that. Also get banner. This lets us pretend we are an LCS top laner. Other core items include righteous glory and mobility boots. Sunfire is also pretty cool, combine it with trample and waves die so fast. Don't build ad or ap, now that you can't w and auto you can't abuse triforce as much. If you really want a sheen, go iceborn.

Alistar: Bestest Tank.