I think, arguably, metaspace. If I remember Corwin's little teaser explanation from back in S1M2 correctly, along with other pieces gleamed from later pieces, I believe the higher warp factors are faster than metaspace, but take a ton more power to reach. Metaspace is quick, but unless you're installing a metadrive, you need to use the gates. Although honestly, since metaspace is so fubar in how it maps out, I think it's just a question of where you're going and where you're starting from...