What is the best soft shell for cross-country skiing?
Kinda depends on your price range, Paul. You can spend a little, or spend a lot.
At the lower end, a decent soft shell can be had in The North Face's Apex Bionic ($129). It uses a generic polyester fabric that has some stretch along with good wind resistance and a little bit of insulation. A durable water-repellent coating sheds snow and light rain, and it's cut to layer over heavy long underwear. REI's Neo ($109) is similar, with a more "around town" look for use during the week as well as on the weekends.
In the midrange, a good choice is Cloudveil's Serendipity ($200). It offers a lining that blends polyester and wool, along with a more breathable shell. Other nice touches include a hem that can be tightened to keep out snow, and chest pockets designed for easy access when carrying a pack. This is really what a soft shell is all about: a versatile piece for moderately aerobic activities in nearly any weather condition except hard rain or driving snow.
At the high end you have Arc'Teryx's Gamma MX ($379). This is really a serious piece, with Polartec PowerShield fabric—maybe the best soft shell material out there. It also gives you watertight zippers and a really nice cut for good fit and flexibility for nearly any activity. It's the soft shell for harsh conditions. Layer it over some good Icebreaker or Smartwool mid-weight long underwear ($75) and you can brave pretty darned extreme stuff.