Is Tanjiro Demon the Strongest?

1) Demon Tanjiro

It is important to note that Tanjiro also received every last drop of Muzan's blood, which was way more than what any other demon from the Upper Moon had. As a result, he became the strongest demon in Demon Slayer.

Is demon king Tanjiro the strongest?

To add on to this you can see that Tanjiro is also the strongest Demon Slayer of his time because he can now use Sun Breathing PERFECTLY after learning from his visions.

Who is stronger than Tanjiro Demon Slayer?

Nezuko would be far stronger than Tanjiro in the base form in terms of pure physical abilities. But Tanjiro gained superhuman strength and speed owing to the activation of the Demon Slayer Mark. Comparing the two in these forms, Nezuko might still outperform in terms of speed and strength.