There aren't any that can survive that cold with out a little protection.

I would try
Cycas Taitungensis
Dioon Edule
Zamia Floridana

I don't mind incorporating a little protection, I just don't want to have to build structures and use supplemental heating. Building a leaf cage around the plant and filling it with leaves is really all the protection I care to use to overwinter one. I like the look of both Taitungensis and Dioon Edule, but I assume that Taitungensis might be the best for the job. I read that it's the fastest growing cycad. I also read Cycas Panzhihuensis is really cold hardy, but I have no idea where I could get a sizeable one. Taitungensis seems like it might be pretty common???
Thanks

Palmpunk, try reading up on the Nova Wilialak. I bought one of these from Robert Chumley last spring. From my personal research, I found that this could possibly be the most cold hardiest trunking Cycad. It also is one of the fastest growing. The only problem (not a problem for me, but maybe for you) is that the trunk is actually a grey or white trunk.
Anything you buy, call Robert Chumley, he will take care of you.

Some species and reported cold-tolerances:

Cycas panzhihuanensis, 0F
Zamia integrifolia, single digits
Cycas taitungensis, low teens
Cycas revoluta, mid teens
Dioon edule, teens

Generally, you are more likely to have plants survive these temps if they are healthy, established, and on the dry side. Leaf dieback is expected, and will most certainly slow down growth each year considerably.

It's one thing for a plant to be able to survive a certain amount of cold, it's a different situation entirely if the plant has to cope with it on a regular, yearly basis.

I wonder if in the long-run, in a zone 6 or 7, the end result is a weakened specimen.

Peopl have already given some good suggestions. For longer term freezes, the best cycads are the ones with subterranean stems will survive lower temperatures because the ground temps will always be warmer than the air temps. As an example, I have seen the Zamia floridanas at Steinhatchee where the apecies were 12 inches below the surface, which means it would have to get about 15F a foot below the ground to kill them. A really good plant that hasn't been mentioned yet is Ceratozamia hildae. The ones I sold to Zilkor BG in Austin took 12F without any leaf damage, which means they could easily survive single digits if the plants were planted deep enough. Also, for what it is worth, ANY pumila type zamia would be like Z. floridana, so if that would make it, you have another 10 species to choose from.
Oh yea, for the comment on trying to find sizable Cycas panzhihuanensis, the first commercial seeds were brought in from China in 1995, so there are people out there who have them available for sale with up to 1 foot of trunk, so availability is not a problem, what WOULD be the problem is what you are willing to pay for one, because they are truly easy to find with a little looking around.

I understand that Ceratozamia kuesteriana is another low temp cycad. I've never protected mine down to 27F. I did note that it prefers shade, dry in winter, moist in summer.

Thanks everyone for all of your helpful suggestions. Thanks cycadjungle for your expertise, especially for filling me in on Panzhihuaensis. I figured I was out of luck on finding anything other than a seedling. I'll take a look around and try to find some of these species.