For the smoothest contact-based/conventional switches of the bunch, you'll probably be looking at Tealios (whatever the latest version is right now) or
Geekmaker. I only have experience with Tealios V2 so far but I can attest to their smoothness, they are pretty much the top dog even without lubing. As for Geekmaker, some claim they are smoother than even Tealios stock, but I can't vouch for that since I never tried them before, but they should be on par but at the cost of consistency.
But if you're willing to put in some work (lubing and
Cherry hurrhurr picking) you should probably skip the boutique switches like Tealios and just get Geekmaker or other more affordable linear since you can bring them up to near stock Tealios smoothness handily with better acoustic than Tealios at considerably lower cost.
You can consider Optical switches like Flaretech and Gateron Optical as well but keep in mind they are highly constrained by platform and will probably have very limited customizability; seeing that you're planning on a build, this is probably not the best choice.
The smoothest linear switch is probably a cherry picked vintage, however gateron ink with the right lube / spring is second to none and the most consistent. Although Sound isn't the best, it's pretty good.
If you have to lube it, it's by definition not the smoothest, and especially not most friction-free .
Sorry guys, no matter what reddit hack job thing is in fashion now (inky zealous pandas?), it simply will not beat a good contactless switch. Heck, I think they might have a tough time even beating FLS (if you can find one NOS).
I've started to think subject of contact switches vs contactless ones are not as black and white as it is, I've recently acquired a Zowie board with Flaretech A and while they are smooth, there really is not much noticeable difference between them and stock Tealios, and Tealios pretty handily beat them in off-center smoothness. So unless Flaretech A is considerably worse than their B iteration, the result is not what I'm expecting of a contactless switches.
So it got me wondering just how much improvement in tolerance and housing/slider smoothness of a top-tier contact-based switches can compensate for additional friction point from the contact? (ie: contact switches with good housing/slider vs contactless switches with average housing/slider) Given that in MX switches, the area of friction between the the slider and contact are still relatively low compare to the area of friction between housing and slider and pretty much all noteworthy contactless switches in the market right now are essentially still just MX design with different working mechanism, it'll be interesting to see people gauge the actual difference between the smoothness brought about by improved housing/slider vs lack of friction from the contact.