Keep in mind, the OP had 4 possible evaluations of Durkon's power:

1. More powerful than Malak
2. More powerful than Tarquin
3. Most powerful melee character
4. 2nd strongest character after Xykon

It's not necessary that they all be true, and there is some room for argument about exactly what most powerful means. It is possible, perhaps, that Vampire Durkon is the most powerful "melee" character without meeting the other criteria.

That he is more powerful than Malak seems probable, but his current condition probably limits the chance of his potential superiority being displayed, somewhat like Xykon's control of MiTD, who may not be more powerful over all than Xykon, but is feasibly more powerful in certain areas.

More powerful than Tarquin is more difficult to judge. We don't really have any way to put an upper limit on Tarquin's abilities because it's unclear that we have ever seen him fully invested.

Further, even if he is more powerful than Tarquin that doesn't guarantee he is more powerful than Redcloak, though if we are limiting the discussion to power as a melee character, perhaps it would follow that that would be the case.