Best Villain? Part 1: "C" Villains

A common senerio in the Sonic series is Dr. Eggman finding a monster and being betrayed by it. Some might even consider it a running gag. This mainly started in Sonic Adventure, with the water beast, Chaos.  

Chaos was first a peaceful creature--and a friend of Tikal--but he knew only greed and anger from the Knuckles Clan and from Dr. Eggman awakening him. Gathering power, he transformed in a rage--intent on wiping out everything. Tikal urged the heroes to help lock him away, as she had done 4000 years prior, but Sonic knew that would not solve the problem. Sonic took on the monster himself to "open [it's] heart." With Tikal he was able to show Chaos that power can be used for good and that there were many who wanted peace rather than destruction.

Chaos had a lot of potential, but he doesn't talk. He doesn't really have expressions, either, making him more of a beast. Unfortunatly it's only the other characters--Tikal, Sonic, Knuckles, etc--that help peace together Chaos' motives and "personality."

While the idea works, Choas is overrated and actually kind of flat. If he were swapped with some other "monster" character it would not make much of a difference.

Many other flat, "monster" characters would follow suit: Biolizard, Babylon Gardian, Iblis, Solaris, Ifrit, Dark Gaia, Time Eater, etc. 

They are not all pointless. The ultimate life prototype, the Biolizard, for example, is a sharp contrast to Shadow. The Guardian shares physical similarities to the Babylon Rogues, hinting at their past. Iblis is the power while Mephiles is the cunning. And Dark Gaia is an all out powerhouse that can even harm Super Sonic.  

Though they all have different aspects, it was Chaos who originally set the stage for these enemies and how they should be portrayed. 

These types of characters help raise the stakes and bring in new ideas. Still, due to their nature, it's hard to call them a "great villain." They just don't have the spark, the personality.