Every Sonic cartoon has made the mistake of making original characters part of the core cast. The Adventures of Sonic the Hedgehog could be considered an exception to this rule, but even then they changed the designs of all the villains.
All of DiC's Sonic cartoons pretty much did their own thing and slapped Sonic's face on it.
Sonic X was generally accurate in terms of character designs, personalities (though Eggman was played for humour too often for my tastes) and other such details but played up the ensemble cast element and shoe-horned in the awkward "stranger in a strange world" angle.
The Sonic OVA two-parter was the opposite of Sonic X in many ways. It got a lot of the details completely wrong (Knuckles, South Island etc) but the tone was mostly right, very fast moving and action driven with Sonic always in the spotlight.
Tails was particularly well handled, they utilised the boy genius aspect of his persona but didn't overlook the fact that this little kid is Sonic's sidekick and joins him on his adventures. With the possible exception of Sonic Advance 3 none of the games have gotten this part right since Sonic Adventure.
I'm still of the opinion that an animated Sonic would work best as a series of action driven short story music videos though.
http://www.youtube.com/watch?v=19qdCEPFz4w
Imagine that kind of thing with Sonic and a wide variety of music.
