Keycool 84 is best for keycap compatibility. It has standard sized spacebar and 1.25 and 1x bottom row caps. The only odd cap is a 1.75x shift and most sets with a Tsangan option will include this key. You may have to make do with incorrect legends on the right hand side modifiers and edit keys, though.
Choc mini has nice default keycaps (thick POM), but is more of a PITA to find caps for due to having a 5.5x spacebar.
The Race 2 has a standard spacebar size, but weird sized Esc and Del which are practically impossible to replace since they're top row keys. If you don't mind having 1x keys on those spots (and gaps left and right of the keys) you can find sets that will fit, but you'll need to find extra 1.25x keys for the arrow and edit cluster. Unless of course you're okay with gaps there, too.
So, I'd get a Keycool 84 II, with genuine MX Browns (the latest version of the Keycool 84 uses Kailh switches) if you can find one.
