For static tripod shots, the usual answer is: use your camera’s native/base ISO for the lowest noise. On many cameras that’s ISO 100, but some models use ISO 200 or another value, so check your manual.

If your subject is reasonably well lit and exposures are only in the low single-digit seconds, base ISO is typically best and sensor heating/dark-current noise usually isn’t a major issue.

There is one caveat: with very long exposures, noise can build up over time. In those cases, a somewhat higher ISO with a shorter exposure can sometimes produce less total noise than staying at base ISO for a much longer exposure. The provided answers suggest this only becomes relevant once exposures get quite long, not for ordinary tabletop/product shots.

So the practical rule is:

• use native/base ISO by default
• only consider raising ISO if the required shutter time becomes very long
• test your specific camera if you need a precise threshold, because it varies by model

Also, tripod use alone doesn’t guarantee maximum sharpness—use a remote release, live view/tethering, and avoid vibration sources.