From what I've been able to work out, and I am talking in generalisations. Happy to be corrected though...
If you want the fastest bike, then you'll choose between the three Spanish MXers, and the YZ250A. You'll pay good money for the bike, and live with relatively difficult to find/expensive parts 'cause you know you've got the best bike on the grid.
The northern European MXers are half a small step behind in speed (ie: less than most of us will really appreciate), but generally a bit easier to live with in terms of durability.
The Elsinore is the next best bike, but the gearbox is A Big Worry, both in terms of breaking and getting replacement parts.
Then the TM250, which kinda seems to be a forgotten bike, so its probably a bit under-valued relative to how good it is as a race bike.
And then the MX250A, which is still worth considering because its so easy to live with - bomb proof, easy/cheap parts, etc.
I don't know where the KX fits in, 'cause you 'never' seem to hear about them. I know I don't have the patience to deal with old Kawi parts availability hassles for a race bike, so I haven't looked deeper.
Personally, I've got a DT250A that's basically at MX specs and a "DT175MX", both of which speak volumes about my ability/goals as a racer....
If the money was available (or I had the talent to justify it) then I'd have an Ossa Phantom and/or a YZ250 for sure, with the Montesa and CZ being seriously considered.