
Wilson do claim long distances for their Di7's, but that's 'cos the lofts are all strong. I.e. a Di7 8-iron has the same loft as typical 7-iron from another manufacturer. So who's kidding who?!
Your question should really be: which is the longest iron with a (for e.g) 21° loft (typical 3-iron)?
But you might find that it actually more dependant on the shaft (type, flex, torque etc).
I would hazard a guess that some of the "hot" face irons such as the TaylorMade r7 XD etc would be amongst the best, if all other variables such as shaft, loft etc were the same.