There's a sticky for this. But here. Though strongest seems to imply power, which Luke does have over all the other Jedi Vader had fought.
A warrior needed to contend with equals. Obi-Wan was gone, and the other Jedi were all extinct, save one, who was the strongest of them all. His own son.- Shadows of the Empire.
Originally posted by Rockydonovang
that quote would imply kenobi was equal to him
The quote also implies Jedi, in general, are his equals. The quote most likely refers to the fact Vader didn't have any other trained Jedi to fight against at that point and Luke was the strongest of all the Jedi he fought.