Ardent censer on Alistar??

You could. I would only do so if the majority of your team is AA reliant or low on damage. Honestly though, most of the time you just want more tanky/engage stuff for Alistar.

Ardent Censer is not a bad item for someone like Sona/Soraka or even Karma if they want to help a specific adc like Vayne or Kalista or have players like Irelia on their team. The reason why Alistar doesn't usually get this is because his heal range is quite low and is not targetable as well as the base stats (AP etc) are wasted on him compared to other more preferable items.

Zeles herald is probably a better choice on alistar.

The mana is enough for him, he doesn't really require regen all that much. The armor is better than the ap that ardent would give. The damage you team gains is most likely going to be higher with zekes. The CDR provided is the same as well.

Overall zekes is better offensively and defensely.

Anyone with a quick to apply heal/shield, especially AoE, benefits from Ardent a good bit.

There's better out there. It's not as short a cool down as other healers.

Ardent was good on Alistar before the patch when it affected Minions. Alistar is known for great pushing, so that effect synergized really well.

However the item itself is not that well suited for Alistar and even the teamwide damage buff doesn't make up for that.mm

tried it, it is mostly a gimmick and doesn't work nearly as well now that it doesn't buff minions.

Before that change it was a splitpushing wonder item.