Cold hardiest are kumquats and of the Mandarins, I believe Satsuma. Kumquats are not only cold hardy, they are also much less prone to bug issues in my experience. Being small, you can fit them into smaller areas, and maybe add more trees. Skins are edible on these, and sweet too.

Thank you, njoasis. I would like to get a kumquat. That's a good idea. Any other ideas on that can handle down to 20 degrees or so?

If out of hand means it is no to tart that it chews your lips off, then meiwa is the best kumquat and then fukushu comes next. chose your rootstock carefully.

Here is a link that might be useful: http://fshs.org/proceedings-o/1998-vol-111/180-182%20(FERGUSON).pdf

Flying dragon or poncirus trifoliata would be your best root stock. Most large nurseries are using a fast growing root stock. They get a nice tree up in a short time but it will make a large tree and cold tolerance is lost.

Wow... that's a great article on rootstocks for kumquat and calamondin. I have a lot of citrumelo rootstock right now and I thought it was a generally good rootstock for hardy citrus, but that article suggests otherwise (for kumquats and calamondin). I would order from Four Winds Growers. They say they use dwarfing rootstock, but I am not sure what exactly.