I had several dahlia plants accidentally overwinter in 5/6 zone this year... Just depends on the year, and how much moisture is in the ground while tubers are dormant. Check out this thread: Do I need to dig my dahlias? Pay specific attention to tarping, for best overwintering results.

Your question is about which varieties overwinter well in the ground, but I would suggest changing your search to great tuber producer/survivors. Lots of beautiful varieties out there are hard to overwinter in the best conditions, but we dahlia fanatics keep on doing it because they win at shows, or they won our hearts with their beauty... Focus on tuber production and storability, and those will do best in the ground year-round (still should dig/divide every year for best results, though).

Check out Swan Island's list of varieties. They typically don't keep around finicky varieties that need pampering. Also, you can read this thread compiling a List of AMAZING tuber varieties.

Hope this helps.

CiCi