Is Gintoki's sword one of the strongest swords?
Gintama is primarily a comedy gag manga, so it's wise to not take everything seriously.

The main reason for Gintoki having a wooden sword is so that he can walk around Edo without getting arrested for illegally carrying a sword. It also keeps the series somewhat light-hearted because he does not often cut people up with it. But despite it being just a wooden sword, he uses it to smash everything from heads to giant alien spaceships. Gintoki himself is sometimes depicted as being on even footing with Alien species which are supposed to be far stronger than any human. This apparent strength of Gintoki's wooden sword is a good example that highlights the gag nature of the series. 

To answer your question though, it is shown at one point in the series that his sword has broken on multiple occasions, and each time that happens Gintoki just buys another one from teleshopping. So the swords themselves are definitely nothing special. 

I don't remember exactly when, but at a later point they showed an episode in which it was revealed that a spirit actually resides in his sword. So if anything, its power can be attributed to that spirit.