being probably one of the more skinny folks here, I use all kinds of layers depending on the conditions, but I particularly fond of my merino wool long sleeved top with a high neck. Thin wool top that gets used probably more than any of my various layers, all through cold to cool weather riding and cool to cold, so march to may, and sept/oct to dec.
I just happen to like how it works and feels, but use various fleece and polypro stuff on top and in conjunction with.
I guess I just happen to be in the group that finds that if doesnt smell as much as my polypro stuff after days of commuting, so prefer to use it as a base layer (plus it is so tight fitting anyway it really is a base layer only, I just put the other synthetic stuff over it)