I think an HSA would be your best option due to the tax benefits but that is only possible if you have a qualifying high deductible health insurance plan. HSA savings accounts are tax deductible when you deposit the money and they are tax-exempt when you pull the money out for health expenses so they are actually better than either a traditional IRA or Roth in that they are tax free at both ends rather than just the deposit end (traditional IRA) or withdrawal end (Roth). But if you don't have a high deductible health plan available or have reasons not to use one then it is a moot point.
If you haven't maximized your tax advantaged space (IRAs, Roth, 401(k) etc. that would be your second choice of saving vehicle for retirement health expenses. Traditional IRAs you can take out a 59 1/2 and Roths you can take out any time (just the deposits, not the earnings).
You would only want to consider taxable accounts if (1) you are contemplating needing the money before you are eligible to take it out of your IRAs, or (2) you don't any more tax advantaged space.