Solaris isn't the strongest Enemy of Sonic, not even close.

Or to be more specific, while immortal, Solaris doesn't have enough strenght. Alot of people keeps saying than Solaris is the most powerful being of the Sonicverse, and how people keeps using him as evidence of how poweful Super Sonic is, when actually, he shouldn't be the one used in that kind of examples.

In case you don't know, Solaris is the fusion of Mephile and Iblis, this two are completly immortal, Iblis is the strongest part, since he destroyed the entire world with his mere existence, and Mephiles not only can go to the past or the future, he can create infinite clones of himself. To put it simple, the only way you could defeat this two, is to seal them, just like what happened with Blaze going to another dimension with iblis, and Shadow with the Scepter of Darkness.

However, once they are fused, Solaris can control time and space, destroy different multiverses, he is capable of ignore the invulnerability of the Super form, has a barrier who not even Super Sonic can't surpass (althought he can't attack either), he can upgrade himself, giving him more strenght than before, and you can't kill him, unless you can destroy him at the same time in 3 different eras, the past, the present, and the future. The only way Solaris was defeated, was thanks to Super Sonic, Super Shadow and Super Silver destroying him at the same time in 3 different eras

Now, after hearing this, you might be wondering how the hell this guy isn't the strongest character in Sonic universe? Because that place goes to EggWizard

EggWizard is the final boss of SonicRush 2, a robot created by Eggman and Eggman Nega, how this robot is capable of surpassing the god of time? Basically thanks to the Jeweled Scepter, EggWizard not only has the power of the stars, but also he can take two super forms at the same time.

"But Solaris needed 3 Super forms", no, he only needed ONE Super form to get destroyed, Sonic Shadow and Silver weren't fighting together in that time, and the 3 of them defeated Solaris alone, EggWizard was fighting Super Sonic and Burning Blaze, and even Sonic said they had no chance of defeating him alone.

EggWizard can fight at the same speed as the Super forms, he can throw huge missiles capable of covering the entire DS Screen, so basically, he can cover all the space he wants, he can create fire dragons, lighting balls who can turn into laser beams who can paralyze a Super form... This are only attacks, but the feats are even more impressive.

EggWizard can manipulate the reality itself, he can destroy entire planets without any problem, he can even fuse dimensions, or destroy them, so obviously he can also survive that kind of attacks, the power of the stars is also canonically stronger than the Sol Emeralds and Chaos Emeralds, hell, a fraction of the power of the Stars is capable of destroying a entire kingdom like it was nothing.

To put it simple, while Solaris might be undestructible and in a fight, EggWizard couldn't destroy Solaris (probably), Solaris couldn't take one single Super Form, while EggWizard was capable of taking two of them at the same time, his power is definetly higher than Solaris, and we don't really know the ultimate power of EggWizard since he was just toying with... You know, a entire dimension.

So yeah, Solaris is unstoppable, but definetly isn't the most powerful being in the Sonicverse. A fight between this two would be basically impossible to end, not because they are equal, but because Solaris being immortal, however, considering the feats EggWizard showed, he probably is capable of destroying Solaris in some way.