Needle palms would definitely be the best. There is a great difference in cold tolerance among palms of this species dependent on the age of the palm is and where it was grown. I would definitely be willing to spend the extra money on a larger palm from a state like NC, SC, or GA because they would be much more cold tolerant than a greenhouse grown specimen. Some nursuries sell needle palms that have seen temperatures in the single digits.
The needle palm has been known to handle temperatures as low as 0F when mature with little damage. Its a palm that should be used much more frequently in zone 7's and will do well with just a bit of extra protection in your zone 6.
Trachycarpus are probably the next most cold tolerant in my experience, but if you live where summers are warm (not in the pacific northwest) then Sabal Minors would also do great for you. Sabal minors are a bit less cold tolerant than Needle palms and they can get damage below 10F, especially when younger, but they can come back from cold temperatures if your summers are warm. Trachcarpus can handle much cooler climates so if you live in the Pacific Northwest, then that would probably be the best palm for you, along with needle palms. Trachys do great with hot summers also so they can grow well in the eastern states too!
Heres a list of the few palms that I think could survive a zone 7 without protection added (a sheltered spot would be best and record cold winters could kill them if they arent given protection)
- Trachycarpus fortunei
- Trachycarpus wagnerianus (I wouldnt try it without protection)
- Trachycarpus princeps (I wouldnt try it without protection)
- Needle Palms
- Sabal minors
- Sabal lousiana
- Sabal Birmingham

I grow Trachycarpus Fortunei, Sabal minor, Livistonia, Butia, and Mediterranean fan palms with protection, but no problem at all.

Good luck!
-Alex

Might as well get the hardy palm you like as you will need to protect any palm you plant.

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Alex, I have warm summers where I live, so sabal minors would be a good choice.

In my own experience, windmill palms work best, but do need protection. They can handle more cold than most palms, grow pretty quickly so they can recover from damage, they're relatively inexpensive, and they are trunking. They do need protection every winter, but I uncover mine as early as the beginning of March, and still haven't covered mine yet this year. Probably won't cover them until the beginning of December, depending on temps. Needle palms don't recover very well from bud rot, and neither do sabal minor, in my experience.

In zone 6b/hot summer climates, Needles (Raphidophyllum hysterix) would work well. Might to best protect even them for the first season or two. Though, mine went through last Winter unprotected for the first season in the ground and survived. I must say, I am liking Needles much more than I used to. I have a nice clump developing in on the south side in the middle of a lawn area. They also put up with extreme heat in areas where that is a consideration. Also, I notice they really grew much faster in the ground than in a container. Unprotected too, I have Sabal louisiana--which does eventually develop some trunk even though it's probably considered 'trunkless'. (I have to say, over the years, my aesthetic appreciation for the trunkless species has increased.) You can also grow, with some protection, Trachycarpus fortunei (Common Windmnill), and Trachycarpus wagnerianus (Stiff Windmill).--Common Windmill is faster growing though. And depending on you soil conditions, drainage, protection,etc.., you might want to try a Chamaerops humilis (Mediterranean fan). They are fairly common, robust, and clumping. In a very bad winter you might suffer the loss of a few individuals but bet the clump would survive. But it would be important to protect the Med from winter wettness--an advantage for just about any of the palms.