Like Ridwanul said: power is something very different than strength by itself, power can be even social, if you have a bunch of tough guys willing to protect you that can be power; obviously not the kind of power we are talking about here, but well, just to show that there are strict and brand definitions for the word "power" and that is useful to define what kind of power you're talking about.
But Nero seems to be much more stronger than Dante while talking about physical strength,
he was able to smash the Savior's (statue) face with "only" his devil bringer. But when it comes to fighting skills and diverse ways to use demonic power Dante is miles away from Nero: he can use his demonic power to move faster (not only in DT mode, Sky Star serves as one example here), he have a very tough body that can be severely damage without killing him, he can fly while in DT mode with some specific weapons (Sparda or Nevan), he can even summon thunder with his hands while using Alastor, his DT form expand even more his abilities (he can use more demonic energy to use more Sky Stars, for example) and he can use the powers of other demons along with his own (when he uses a devil arm, he gains access to some specific moves and in DMC3 and 1 the devil arms even change his devil trigger form). And the one who implied that
Dante was even stronger than Sparda was Griffon in the first game, you're right on this.
I don't think Sparda was that "all-powerful" demon everyone thinks off, i think that Sparda's power is what we see in the Mundus fight in Devil May Cry , Dante was using Sparda's power that were sealed in the sword in that time; that was Sparda's demonic power, he's not that powerful if you stop to think about, his "power" (what make him a strong being, even more than Mundus and the other demons) must have the same nature of Dante's powers: skill, the ability to adress very different situations using his power in different ways, Mundus was clearly a more "powerful" demonic entity, he can summon a lot of things including meteors and some demonic orbs that shot some spikes, summon barries made of demonic power, explode stone plataforms just by shaking his hand, give life to other demons (Trish and the lava dragon he summons in the fight with Dante), transform all the background in something similar to space (?) and the cherry in the top of the cake is that he's all immortal even if his physical body is completely destroyed.
But, yeah, i think that Dante is more "powerful" than Nero because he seems to have more demonic power and traits. And we already seen him beating Nero before, but i think Nero has the potential and the tool to surpass Dante even if he's more limited (he seems to need Yamato to unleash his fullest demonic power, for example, although i'm not certain about this).
