The best tach I have found is the MasterTach, built by Allen Booth, and sometimes available through
www.pspec.com
It is an analog tach and runs close to $200
They have gauges for different rpms ranges, and are extremely accurate at higher rpms (I run them over 23,000 rpm, I believe the top gauge goes up to 30k)
Having an analog (sweeping) gauge is much more accurate to me than to see numbers jumping around on a digital display. These tachs are not for everyone, but I would say they are better than the TNC, and I have tried them both.