Blis M18 and K12 strains. There are many brands using these. I prefer pills straight from Blis or Life Extension.

If your mouth has an unhealthy microbiome, you might need to place a lot of emphasis on killing bad bacteria initially. For this purpose, salt water or mild antibiotics (chlorhexidine) for 2 weeks combined with double dose of probiotics should do the trick.

Obviously, sugars feed bad bacteria, which metabolize it into acids to erode your enamel. Avoid entirely.

I reckon milk kefir is pretty good, apparentiy it has a particularly broad and strong co-operative of micro-overlords, ahem, micro-organisms! Dental-protection? probably? No studies to back that, but in my opinion, it's one of the best cultures for your insides.