Which game's A.I. engine best suits Skyrim? Please don't say "Radiant"...at all.

Yeah, especially because there is such an ambundance of alternatives to Radiant.

It will be very likely Radiant Mk 2 - unless they decide that their AI concept just doesnt work.

It will be an adaption of Radiant, as that is what has been in the works for a while. Just like the render engine and everything, it is all an evolution of what they started working with back with Morrowind. And for the record, Fallout 3 and NV used the same AI as Oblivion, just vastly more improved.

Will there be improvements? Undoubtedly.
How much? Only time can tell.