Eyebrow razor. Believe it or not there's some real good sharp ones you can find online that don't have the ridges. Just read the Amazon reviews and if people are replying stuff like "this got me to beans in one swipe" or "shtwt this one is for you guys" (self harm Twitter slang) then you know you're looking at a fairly sharp blade that other self harmers are using with success. These cut just as well as a double edged razor blade, however since they have handles it's way easier to apply pressure, plus you don't need to worry about your fingers getting sliced since they're one sided.
A fair warning though: if you're going to use sharper blades and go depth chasing, make sure you're at least prepared and have the proper medical supplies to bandage deeper wounds. Having non stick gauze pads, medical tape, and steri strips are a must.