Very vague question. What are you most looking for? Low voltage drop? Aesthetics? Do you want dual or single 18650? 18650 or 26650?
Me personally, I always went with copper dual 18650 in parallel. I have an Osmium and an Incubus. The copper has low voltage drop and has a nicer look to it. Dual 18650 in parallel for the longer battery life. If you're looking to build something that has a really low resistance, I suggest using a dual parallel 18650. I really like my Incubus when I want to use my mech mods so thats what I would suggest. Just keep in mind that the incubus is a little hard to clean out because of you have to disassemble it.
Aside from my two cents, Mech mods generally function in the same manner as the next so it's kind of a tough choice to make for someone else.