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For models that are produced by GW or forgeworld it's the Tau Manta, followed by the Reaver Titan in distant second place.

For models that are produced outside of GW, there is this: http://www.tsipisgames.com/shop/warbringer-maximus-mkii-no-weapons-free-shipping/ But I've not seen one in person to attest to the quality. I've heard decent things though.

If you're asking about points and power alone without regard to existing models, then everybody else is right about the imperator. Although I have heard that Will Hayes (their best designer) at Forgeworld would like to eventually create a warlord titan. Knowing Forgeworld prices I'd be surprised to see it under $1500 though.