(I don't really know the Japanese voices as much as I do with the dub voices. Also, for better understanding, most reviews will be fairly short.)
Dub Sonic only sounds a little like Japanese Sonic, sounding more like an extra cocky Thomas the Tank Engine rather than a more serious and mature man with only a tinge of cockiness.
Dub Tails sounds pretty accurate to Japanese Tails except that his female VAs' dub voices are nothing short of throaty when he yells.
No dub Knuckles sounds like Japanese Knuckles at all, sounding more like a generic dude rather than sounding like Spyro & Gilbert Gottfried having a funky baby, particularly when Japanese Knuckles raises his voice.
As bad (e.g. Adventure 2) or inaccurate (e.g. Frontiers) as dub Amy's voice is, Japanese Amy's voice is too annoying and is just like an anime girl, betraying her determined personality Lisa Ortiz & Cindy Robinson reflect.
Dub Shadow's dull monotone voice, while arguably the most fitting voice for him ever, is hardly like the rich and older voice of Japanese Shadow who sounds more like a hero than an overly serious person.
Despite being the most accurate, Karen Strassman's Rouge is only half accurate to Rumi's Rouge due to her squeaking voice and even Karen's other roles sound similar probably because it's close to her real voice.
As bad (Adventure 2) or inaccurate (Frontiers) as dub Amy's voice is, Japanese Amy's voice is quite annoying and is just like an anime girl, betraying her determined and cocky personality Ortiz & Cindy reflect.
Dub Shadow's dull monotone voice, while arguably the most fitting voice for him ever, is hardly like the rich and older voice of Japanese Shadow who sounds more like a hero than an overly serious person.
Despite being the most accurate, Karen Strassman's Rouge is only half accurate to Rumi's Rouge due to her squeaking voice and even Karen's other roles sound similar probably because it's close to her real voice.
Dub Big sounds like almost just like his Japanese self, the only difference is that he sounds just like Hugo the Abominable Snowman in the dub.
Rebecca Honig's Cream sounds just like Japanese Cream except that her voice sounds like quiet wheezing when not happy and doesn't have a unique cry while Japanese Cream has one and does not wheeze-shout.
I'm starting to agree with other people, dub Vanilla sounds awful, sounding just like Granny rather than a normal woman like in the sub.
While all of the dub Vectors sound similar to the Japanese version, all three of them sound goofy. The closest sounding is Dan Green's Vector.
Bill Corkery's Espio sounds silly while Matt Mercer's Espio sounds too deep, lacking a younger, dull monotone voice as in the Japanese version.
Japanese Charmy sounds just like a baby, not a falsetto (Sonic Heroes) or a smarty (Sonic X). Colleen does a better job imitating his Japanese voice.
Sticks is supposed to sound like a normal anime girl like in the sub, not a totally unique voice that sounds like someone choking like in the dub.
Jet is supposed to sound like someone who claims he's handsome like in the sub, not some angry jerk with a high pitched raspy voice as in the dub.
Japanese Storm has a generic deep but smart baritone voice, sounding nothing like his dub self who sounds like a dumb cartoon hot dog vendor.
Dub Wave doesn't sound like her Japanese self. Erica's take sounds too cocky while Kate Higgins' take sounds too high and squeaky. Her unique laugh her Japanese self has is completely absent in the dub as well.
All English Silver voices sound too young and high with Bryce's Silver sounding the worst, being nerfed into a dumb repeating coward.
Same goes for dubbed Blaze who just sounds really young, generic, and annoying. I know she's 14 but her dub voice sounds nothing like the deep smooth Japanese voice of Blaze that sounds more serious and Indian.
Dub Tikal sounds too generic (& too old and deep in Sonic Adventure). She is supposed to sound like an anime girl like in the Japanese sub where the sound of her voice changes dramatically when she's not smiling.
Dub Cosmo the Seedrian sounds nothing like in the Japanese sub where she hast the most generic and Kawaii anime girl voice ever.
Dub Chris Thorndyke actually sounds just like his Japanese self.
Mike Pollock's Ella Thorndyke sounds like Eggman speaking in a falsetto while Kujiro gives her the deepest, heaviest and most beautiful voice ever which sounds like Sebastian the crab. In fact, Kujiro translates to whale.
Out of all the Deadly Six, only two of them sound inaccurate in the dub. Zazz sounds like the sleaziest New Orleans man ever while Zeena sounds too high pitched, bubbly, fishy, and nothing like a seductress.
Dub Infinite sounded too deep, young, and quiet, lacking the older but higher pitched voice he had in the Japanese sub of Sonic Forces.
Unlike dub Orbot, dub Cubot sounds nothing like in the Japan sub, having the world's goofiest voice ever rather than a reasonable gravelly voice.
And the last one I will mention is dub Eggman, especially Mike Pollock's take, who sounds nothing at all like Chikao Ōtsuka's Eggman. Pollock's Eggman (ever since 2007) sounds like an angry, sadistic, unlikable and unfunny villain who wants to kill all life while Ōtsuka gives an unfitting Waluigi like voice that makes Eggman sound like a funny and lovable bully who wants to build a park. At least Kotaro Nakamura gave Eggman a less funny and more menacing voice on par with Pollock and Jim Cummings.
Also, here are the actors who could do a better job voicing those characters:
Tom Kenny: Sonic
James Arnold Taylor: Knuckles
Karen Strassman: Wave, Tikal
Jim Cummings: Eggman
Christopher Sabat: Shadow
Erica Lindbeck: Rouge
Kirk Thornton: Silver
Grey DeLisle: Blaze
Billy West: Jet
Bob Joles: Storm
Kate Higgins: Amy, Sticks
Tara Strong: Cosmo
Bill Faggerbakke: Ella Thorndyke
Mick Wingert: Cubot
Mark Hamill: Infinite
That's not all the characters but it's all I will list for now. I'd like for an opinion on this list to see if you'll disagree or agree but I will respect your opinion.
