Voigtlander Bessa 66 is really portable - hip pocket portable and has sharp glass. Although single coated. You will also have to be good at ranging in metric. I also like the Agfa Isolette III. Very sharp, only slightly larger and has a rangefinder (uncoupled). It is a great pocket 6x6. In fact, a late (50s) high quality folder will often make it difficult to justify the size and weight of a TLR or an SLR for many kinds of image capture. Ahhhh but forget wide open - they are Tessars and at f3.5 - you better stay at f4 or above except for maybe the Certo 6 which has a f2.8 lens on a folder. Rare but it has a great reputation.
I too adore 6x6 folders but they do not at all compete with Rolleiflexes in sharpness and resolution of detail. They fall short for many reasons. Part has to do with lens design -- most focus with moving front elements, so you lose some precision there. Another is that the orientation of the lens to the focal plane is less precise, because the lens is set on moving arms in the folding mechanism. Still another is build quality -- while good on some folders, none approaches the precision found in the Rolleiflex bodies and lenses.
Folders are fun, but they do not come close to the visual acuity of the Rolleiflex. And since the OP asked which TLR provides the
sharpest lens, there is no reason to throw folders into the discussion.
Among TLRs, Rolleiflexes offer state-of-the-art lenses, and they are the smallest, lightest, and best-built among the TLRs. Among Rolleiflexes, the differences among lenses are more imagined than substantial. Planars and Xenotars are identical. So are Xenars and Tessars. The latter show some minute loss of clarity at the extreme corners at wide apertures. If you plan to be shooting wider than f/4 all the time, it might be worth opting for the Planar/Xenotar models over the Tessar/Xenar models. But user error will affect image quality far more than the lens on the camera.
Go for a Rolleiflex in good condition at a price you can afford and don't sweat the name on the lens bezel.
RFXB