I always go for zip neck base layers over the non-zip variety.
Why? In a word – versatility.
In two words – temperature management.
Over the course of a hiking day, weather conditions and exertion levels may vary considerably. A zippered neck allows you to keep things cooler when you are working hard and/or the mercury’s rising. Alternatively, you also have the option of “shutting up shop” when you are taking a break, cruising along mellow terrain or the outside temperature is dropping.
I’ve always found this added flexibility to be especially useful when hiking in inclement or sub-freezing conditions. In such circumstances, minimizing perspiration is one of the keys to staying both comfortable and safe. This often translates to regularly making small adjustments to your clothing system. Enter the humble zip neck. Every gear tweaker’s dream.
Disadvantages? Zip neck base layers usually weigh a fraction more and are a few bucks pricier than their non-zip equivalents.
In regards to materials, I usually go for merino wool over it’s synthetic equivalents. As for specific recommendations, there are a bunch of good options on the market (e.g. Icebreaker, Montbell, Smartwool, Ibex, Patagonia, Minus 33). At any given time, you can usually find at least one or two of these different zip neck models discounted on Amazon, Sierra Trading Post or Backcountry.com.