I believe interest is added to your income, so it depends on your top tax bracket.
In a TFSA, nada.
Tax-free interest exemption in SA maxes out somewhere around R25k p.a.? Then the taxation on interest works out extremely high as not much in the way of offsets. But offshore in a high interest low tax economy it looks better. In which case USD, Euro and GBP are out in terms of earning decent interest.
There are perhaps a small handful of currencies worth looking at in regions taxing at 5% or lower while offering decent interest on fixed deposits. Not without the inherent currency risks associated with exotic currencies. Not that the rand is without major deprecation risks too of course. Then you still need to consider resident vs non-resident with new SARS legislation.