Vests
The most useful thing in a jersey pocket. A gilet takes the chill off a descent, blocks the wind on an exposed road, and packs down to nothing when the sun comes back out.
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A windproof front with a breathable mesh back is the classic pattern and still the best one — it stops the wind hitting your chest without cooking you on the climbs.
If you only buy one extra layer for spring and autumn riding, make it this.