The one that makes sense to take while you are still on the course. It's a yeast, not a bacterium, so the antibiotic doesn't kill it — which is why spacing doses is less of a worry than with bacterial products.
Breadth is the point here rather than any one strain. Start it once the course finishes and keep going for a couple of months — the microbiome takes longer to settle than the prescription lasts.
Almost everyone buys the probiotic and skips this. Introducing bacteria without feeding them is half the job — fibre is what they live on. Cheaper than the probiotic, and arguably does more.
Pros
Feeds existing bacteria, not just the ones you add
S. boulardii is a yeast, so the antibiotic doesn't kill it. It's the only one of these worth taking while you are still on the prescription.
After → the multi-strain.
Bacterial probiotics taken alongside antibiotics are largely wasted. Start once the course ends, and give it eight weeks.
Throughout → the fibre.
Feeding what survived matters as much as adding more. This is the cheap step almost everyone skips.
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