Having said all of that, the best hammer of the job would be the Grand Marshal's Battle Hammer but we know only a select few individuals are able to use that.
Is there a weapon I'm missing or something? I have Captain Rogers' Polite Knocking Stick but it feels odd.
Edit: I'm gonna expand this to two-handed swords too, as Berith mentioned below. There's a few swords that could qualify, like Reckoning, but the green color throws it off. I wish the blue and gold color scheme you can see on an NPC in Icecrown Citadel. I'm a personal fan of Vicious Gladiator's Greatsword, which could work, but it doesn't scream PALADIN. As mentioned by me below, there is actually an axe that works VERY well for paladins, but... it's an axe. Which is odd. It's Justicebringer, which works pretty well with the standard Human/dwarf paladin theme.