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

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.

Baileyje.......where were you conducting, inquiring minds have to know.

I think the best version Bach Mass B minor to start and branch out from is Gardiner/Archiv, the sound is very good but not exceptional. Unfortunately this is still full price version.

For exceptional sound check the Gardiner/Archiv version of Monterverdi "vespro della beata vergine" this is very impressive recording with massive soundstage.

The Monteverdi "vespers" was recorded in St Marks cathederal in Venice Italy, there are sections featuring vocals from upper balcony of cathederal that literally sound like voices from the heavens......very impressive.

Please keep us posted on your findings. Also Gardiner/Archiv series has other major Bach liturgical works that generally are very good overall choices.