US Modular Sleep System for me (have two now), for most temps the bivy and green bag pack down small (basketball size), and works well, enough room for me. Obviously add in the black intermediate bag and the inner space is reduced a lot (even more if you stuff the sleeping pad inside the bivy), as well as the bulk and weight of the compressed size is increased by double, but that's only really for sub-freezing temps (did that a few weeks back, slept warm!) I love the modularity of it, and the excellent reviews it gets, even if bulky. Just means you can leave the tent at home. Lol