What weapon class is the fastest for a smiter?

A Phase Blade with 4x Shael 2x Ber (for Crushing Blow) will allow you to hit max speed WITHOUT Fanaticism, it frees up an aura, so you can use something like Salvation on Uber Mephisto...

For PvP, a Phase Blade with the Grief Runeword would probably be the best. You can get up to 40% IAS and 400 extra damage. Best of all... it's Indestructible! Think of all the gold you'd save on repairs.

Based on this calculator you should be able to reach 6 frames per attack with Smite using any weapon class, given sufficient attack speed boosts from Fanaticism and item bonuses. I tried using the Falchion (which is a 20 weapon speed, the slowest) and it still showed an IAS level that gave 6 frames per attack.

You can find the weapon speeds for all weapon types at this page. As an example, the Falchion requires level 20 Fanaticism and 109% IAS to reach 6 frames. A Phase Blade (which is a -30 weapon speed, the fastest) can do it with just 65% IAS and no Fanaticism.