Depends if you want a code reader or a diagnostic tool...

IMO you can't go wrong with either a Bosch KTS340 or KTS670. They're both the same unit but the 670 has an oscilloscope. The other units worth considering is a Delphi unit. It's not just the ability to read codes but the information these machines carry, which is second to none.

I've played with several machines ad these are the ones that stand out the most, unless you're talking dealer spec tools or truck tools (which Texa is pretty good).