Most reliable: looking at it, it's probably my BTC 5339R-0 with foam and foil switches. It's one of the few boards I own that's never given me any issues of ANY kind.
Least reliable: Focuis FK-9000. That didn't even work at all brand new out of the box xD .
Seeing a foam and foil in the "most reliable" category is quite surprising to me, although I suppose if the foam has not degraded, the underlying capacitance sensing ought to work reliably.
In the least-reliable camp, did your FK-9000 have Alps switches or clones?
=====
This thread is cross-posted over on DT, and between the two, I am starting to see some trends. It seems that in the least reliable category there are many mentions of vintage Alps or Alps clones -- they got dubbed the "Alpha Romeo" of keyboard switches, which Chyros changed to "Alpsa Romeo". Despite the reliability issues, Alps switches seem to be enjoying a resurgence of popularity. This is apt, because sports cars are such fun to drive when they are up and running, and we keep coming back for more even if the car spends a lot of time in the shop.
The most reliable category seems more mixed, but Topre-switch boards such as RF and HHKB get mentioned frequently. In addition, some have cited Filco and CM products, and I would agree with this assessment. One of my first contemporary mechanicals was a Filco MJ2 Ninja, and I still have a CM QFS. The Filco impressed me as being well built. Likewise, the QFS seems quite solid, especially for the price. And although I could use some of my Model M boards as oversized maracas due to the broken rivets rattling around inside, they still work. Likewise, my very first Soarer Converter project was an IBM XT built in 1984 that worked when I first got it from eBay and continues to work several years later.
This is a non-rigorous set of impressions. After more opinions have been accumulated, I might do a more accurate tally.