The best, well thats debatable. As mentioned by leemark, the legendary Stevens and Billington tx103 SUT's are as good as it gets. But you can only buy them through Music First now in their own device. I owned the Bent Audio SUT's that used the Stevens and Billington tx 103's. Long out of production and hard to find on the used market. Very high on the "cool factor" and exceptionally well made.
If you can afford it, the Ypsilon Moving Coil Transformer is as good as your going to get in whats available retail. Used, almost never find a used one.