Mid Tahm is strongest Tahm
It's true. Build:

Proceed to wreck faces.
Tahm is like a Ryze that scales with survivability instead of mana. At max level ult, you get 1 damage for every 12.5 bonus life added to all your abilities and auto attacks. You will do more sustained dps than literally every other ap caster while having far greater tankyness. Give it a shot before you dismiss it.
Also, the best skill order is: Q W E then max R > W > Q > E. Use W for crazy aoe damage from spitting minions and for the occasional champion nuke when you catch someone.