What exactly do you mean by 'best supplement for hypothyroid'? Supplement can mean many things. Are you talking about vitamins and minerals? Or glandulars? Nutri-Thyroid is a glandular. Or are you talking about thyroid hormone replacement? It would be easier for people to help you if you were a little more specific.
However, whatever you mean, the answer is : there isn't one. There is no 'The Best', of anything. The best is what you need and what suits you. We're all so different, and what is elixir for one, is poison for another.
If you want to know about vitamin or mineral supplements, you have to get tested first, to know what you need and how much.
If we're talking glandulars... well, they don't suit everybody. You'd have to try them to see.
But, as you mention rT3, I suspect you mean thyroid hormone replacement - Levo, Cynomel, Armour, that sort of thing. And, there again, the best one is the one that suits you. But, one would have to see the test results to know if you needed T4 + T3, T3 only, or whatever.
So, I'm afraid, your question is unanswerable, as it stands. Sorry.