Something that is comfy against your skin and deals with moisture to your satisfaction. Something that provides appropriate warmth (or lack thereof).
That's usually some sort of base layer, and may optionally be a mid layer. A selection of both to choose the best for the job in hand is nice.
The biking may well benefit from a longer drop tail, and that won't be a problem walking, where a shorter back can leave an annoying gap on a bike, depending on your riding position.
Pete.
