Cherry MX Linear Grey
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Cherry1) Krytox GPL 205G0
2) Tribosys 3204
3) Tribosys 3203
You can swap 1 & 3 depending on if you value the speed (3203) or the smoothness (205G0).
Cherry MX Linear greys are a mid-heavy linear switch, that used to be commonly found in the spacebar of older boards, especially MX black boards. They're just a normal linear, and have been retooled since sometime in 2016. If you can find them at a decent price, I'd say they're worth. But, they will usually be more expensive than Cherry MX Blacks. For that reason, I'd recommend just getting some nice retooled Cherry MX Blacks.
A thin coat of Krytox GPL 205g0 on the stem will get you a nice, buttery feel.