I see Trumpeter has at least 3 Challenger kits:

00308 Challenger II
00323 Challenger 2 Operation Telic
00345 Challenger 2 Kosovo

Which is the newest/best?

It is not so easy to tell. The Kosovo and Iraq versions are newer and were released at the same time, so they represent the same quality. In Kosovo version you get dozer blade. In Iraq version added are additional sand skirts and other parts used during Op. Telic.
But the original kit (without extra armor or dozer blade) was updated at about the same time as these two were released. This means that if you get the old release, you will have poorly molded and misaligned road wheels, lack of some details on the turret, incorrect attachment of rear idler, engine grilles of wrong size etc.. But if you get the later release, all these problems were corrected. Unfortunately there is no way to tell which version is in the box... So it is much safer to get one of Kosovo or Telic releases, as they still have all the parts to build not uparmored version in the box (minus decals for such version!).

EDIT: One more thing: Trumpeter kits are much cheaper, but if you can find Tamiya Challenger 2 at good price then get it!. It is light years ahead of Trumpeter in quality and ease of assembly (you need a lot of patience and putty to build Trumpy kits).

Pawel