For me, I've found that a Patagonia expedition weight Capilene top is one of the best bang-for-the-buck when I don't have that much space. That said, I'd also carry headgear and at least some lightweight gloves. For me it's a given I'll be wearing wool socks.

FWIW, I pack an Outdoor Research (OR) Ice-9 Helmet Liner if I really want to be slimmed down. It folds flat, puts fleece on my whole head including ears and will fit under my rain hat. It's not very stylish, but good bang-for-the-buck when you want to slim down.

-john