This is only between Infinite and Mephiles the Dark; it has nothing to do with Dr. Robotnik.
In that case Mephiles. Or to be more exact Solaris who he is one half of. Powerful entity that is capable of warping time and space in his complete form and considering Mephiles is Solaris' personality yeah I would say he is far worse. Then again Infinite supposedly escaped and flew off so perhaps we didn't see the full extent of his abilities and may return one day.
Melphiles. Don't really understand why his page was condensed into Solaris as Melphiles is understood to be the physical manifestation of Solaris' consciousness (I. e. his mind). He tried (and succeeded) at damaging the space-time continuum.
People agreed that, since he and Iblis are simply halves of Solaris, it was better to have the articles fused together.
Are they both really diferent? I consider Infinite to be the spiritual succesor of Mephiles in the saga. Their personality and mannerisms are the same. The only thing I found different is that Infinite is more cocky while Meph sticks to operating in the shadows, but their evilness are identical.
Mephiles is far worse. Inifinite was loyal to Robotnik. Mephiles was loyal to nobody and wanted to rule over time and space by absorbing the flames of disaster and becoming Solaris.
He actually was Solaris, but he was one half of Solaris. He actually wanted to merge with Iblis to make himself (Solaris) whole again.
What do you think?