I have a Topre board and a Matias Linear board and I like both of them a lot. I've tried Cherry MX Red and Blue. I liked the Reds, but they were noticeably scratchier than my Matias Linears. I kinda liked using the Blues but the hysteresis of the clicky mechanism kinda kills the experience for me. I really want to try out the other Matias switches on a keyboard and possibly some of the older Complicated ALPS switches, those seem to be pretty good.
My nomination currently goes to Topre, even though they're technically not mechanical. Doh, I didn't read the topic thoroughly. If it's gaming we're talking about, my nomination would probably go to Cherry MX / Gateron Red, maybe Gateron Clear but I've never tried those.