Which is the best recording of Bach's Mass in B mi

Two performances I very much enjoy are:
~ Harnoncourt, with the Concentus musicus Wien on Telefunken
~ Gardiner with the English Baroque Soloists and Monteverdi Choir on Archiv

Both are historically informed performances, but beyond that they are very different from each other.

You may need to be more specific:
Cd or record?
period performance or modern instrument?

If you want "period" performance Rushton has good suggestion with Gardiner/Archiv or perhaps Parrott/Virgin.
There are more modern versions in catalog, but I think Bach Mass especially lends itself to the unique insights/sounds offered by period performance.

If you want a more "modern" instrument sound, then on LP or a CD reissue (Philips) is Eugen Jochum conducting the Bayerischen Rundfunks orchestra and chorus...