Is AP Tahm better than tank?
I mean, he does 70% AP ratio + 80-220 base damage just on his Q which is on a short-ass 6 second cooldown, less with CDR which would be pretty easy to get with Spirit Visage which would be good for his E, his W hits with a whopping 300 base damage + 60% AP AOE damage when he spits out a minion, and 20-32% + 2% per 100 AP(not sure if it sticks to 2% or if you can just build 50 for 1%, but that's what the wiki said so eh).
But, as a tank he does tons of base damages anyway, plus you could build like, a RoA or a Staff-thingy-I-forget-the-name-of(slows people with spells, 400 health, 100 AP), or heck, even a Liandry's Torment with his W and Q for major poke damage. Then there's extra things like Righteous Glory or Locket of the Iron Solari for supportive items.