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1:12 IMO.
1:6 is too big to be manageable unless you have acres of space and lods of emone, and the figures are often horrendously limited in actual poseability due to their insistence on using actual doll clothes, or worse still seamless bodies that will crumble if you don't spend every waking hour massaging them with baby powder.
1:18, whilst in no short supply of accessories, is a bit thin on the ground as far as actual decent, obtainable figure lines go, and if you don't want military styled figures or Star Wars you're kinda SOL, not helped by it not being that popular in Japan (which is weird as you'd think with the limited space that there'd be more than just Microman in that scale). Also they often tend to be limited in articulation or kinda funky looking due to the limitations of such a small scale.
1:12 is IMO the perfect compromise between compactness and manageable size whilst being big enough to be nicely sculpted, well painted and properly engineered. Also, 1:12 is a popular scale for dolls' houses, so there's endless accessories to be had for them.
(Pic related is technically 1:1, sure, but they're roughly the same size as a 1:12 figure.)
