What's your budget? Need to know this to really be able to pick one out. On the top of my head though; Used Lenovo Thinkpads or X1 Carbons (preferably newer generations from a couple of years back or with at least the 4th generation of Intel i7 CPUs) are fantastic. Be sure to have at least 8GB of RAM in it and it should fly with S1. Aside from that it's really a preference of which screen type, screen size, keyboard and touch pad quality, cooling, noise levels under heavier load you want. Again though - Thinkpads with the above basics are rather cool and quiet and will definitely be while using S1.
