Is the Dark Side of the Force more powerful?

As with many questions about the Force, the answer depends on a certain point of view.

Yoda says that the dark side is not stronger. However, you note several feats performed by Sith that are far more powerful than anything we've seen Jedi do with the light side of the force. In addition, there the lightsaber duels. Darth Maul is beaten by Obi-wan, but he kills Qui-gon in the process. And, as DVK observes, when Anakin defeated Dooku, Mace Windu bested Palpatine, and when Luke beat Vader, they all did so by using the anger and aggression of the dark side. This leaves Obi-wan's victory over Anakin and possibly Count Dooku fleeing from Yoda as the only "pure" light side wins, and in both cases the Jedi combatant is much more experienced than his opponent. So, it appears that the dark side is stronger, despite what Yoda says.

However, Yoda is unlikely to think of strength solely in terms of winning fights or destroying enemies. He is probably referring to the ability to influence events, in addition to what could be called "strength of character". Looking at events from this point of view, they seem to support his statement.

Mace Windu's Vaapad fighting style was based on controlling his aggression, and was explicitly a light-side technique.

When Anakin fought Dooku, Dooku went easy on him in an attempt to get him to use his anger, so that he might be turned to the dark side. It worked. I doubt that Yoda would consider Anakin becoming a Sith Lord a "win".

Palpatine and Vader tried the same thing against Luke. He beat Vader in a lightsaber fight because he was supposed to. Darth Vader spent most of his time taunting Luke, urging him to give in to his anger. Even when they were fighting, Vader didn't attack much. Luke's real victory was not attacking in anger and cutting off Vader's hand; it was refusing to give in and kill him. This was a "light-side" act. It let not only to the defeat of the Emperor, but also to Anakin's redemption -- something that the dark side never could have achieved.

Similarly, when Obi-wan fought Darth Vader for the last time, he was not trying to "win" in the traditional sense. He was buying time for the others. He also didn't resist when Vader struck the killing blow. So, in a sense, he lost, but as a result, he was able to continue to mentor Luke, and even provide an example for Luke's eventual decision not to fight.

So, what Yoda told you was true, from a certain point of view.