Defintely the hardiest is Acer pseudosieboldianum (Zone 4). There's one cultivar of this species but I forget it's name...'Sodi no uchi' or something like that...Anyways just the species pseudosieboldianum is just as good.

The hardiest of all Acer palmatum is 'Emperor I'...it breaks bud 2 weeks later than all other palmatums and it also hardens off 2 weeks earlier in fall than all other palmatum's. Perfect for us zone 5'ers in the Midwest.

The japonicum's in my opinion are also a tad bit hardier.
They can use some part shade however especially in the midwest. 'Acontifolium' is easy to get ahold of. I have it here in Rock Island, IL fully exposed with minimal but not much dieback. Like I said however, part sun, especially morning sun/afternoon shade would be ideal.

You might also consider Acer griseum (paperbark Maple). Mine is growing wonderfully here. ZERO dieback!

I'm sure there are others but I'll stop right now at these.

Dax

Thanks for the suggestions. I am ordering on-line from worldplants.com, so Korean maple (A. pseudosieboldianum)and Paperbark are not available from them. They do have Emperor and Acontifolium though. My brother actually lives in Geneva. Not sure how that compares with Rock Island weatherwise, but it shouldn't be too far off. His Crimson Queen has done pretty well, so I would guess that any of the varieties you mentioned should survive.

The shirasawanums do well here in Green Bay Wisconsin as do 'Emperor I'. 'Girard's Hybrid' paperbark Maple from Girard Nursery in Ohio is a beautiful and hardy tree here. I'm also currently testing a palmatum called 'Robinson's Red' also from Girard - supposedly a very hardy strain.

I settled on an A. shiraswanum, Autumn Moon. Hopefully it will do OK, as he's successfully grown a Crimson Queens for years.