I'll second the genuine N3B parka, if you can find it. Sportsman's Guide and all of the Army/Navy stores I've seen only offer the Chinese copy. I got mine from an Air Guard guy who was retiring, and it has been great for the cold snaps that New England has been getting this season. It is terrible for hiking or snowshoeing, however, as it very quickly gets too hot. Great for standing around or sucking on an oxygen mask at 30,000 feet, though.
AZRedhawk44 is right about the M-65 (with the field jacket liner). Warm, but the hood is a joke. I remember seeing a replacement fur-rimmed hood that zipped in, but a quick search today revealed nothing. The M-65 is still a standard-issue clothing item in the Army Guard, but most of our guys prefer the Gore-Tex/Polarfleece combo or the new 7-layer system (if they're lucky enough to have it issued). For your own purchase, however, the M-65 is definitely the least expensive choice. Be sure and get a US-made version, I don't like the Chinese copies that I've seen.