My opinion on voices (Sonic and Shadow mostly), both official and fan VA
Sonic - the official
The voice I'm most fond on is the one of Ryan Drummond in the Adventure games, while I didn't appreciate much how he sounded in Heroes. It was the first time I head a convincing Sonic Voice, after years of hearing Pietro Ubaldi in Italy (Pietro Ubaldi is known to dub cat characters. All male cats in anime were dubbed by him in the '90s'. If Big the cat was in Sonic Satam, I think the Italian VA would have been Pietro Ubaldi. His voice didn't work much on Sonic, indeed right now Sonic Italian VA is Renato Novara, a lovely voice for him).
I was happy Sonic had his voice too, like Mario. Ryan's natural voice is very deep. Deeper than RCS', I was surprised hearing him talk naturally. But he made Sonic sound like a loud and cheerful teenager.
Jason Griffith was also a great Sonic. Not so great in Sonic X, but in videogames he had a strong reminescence of Ryan. Maybe he even sounded more natural. Unlike Ryan, Jason's natural voice is not so deep and i think his voice sounds cute at times. To be honest, I can't choose between the two.
My opinion on Roger Craig Smith's voice is not that great. I don't like how he sounds, not even in Frontiers, his voice is too deep and too serious for Sonic. Some suggested that he should dub Shadow, they even did videos with Shadow with RCS voice when he dubbed more mature characters. But, personally, I think he is a bad Sonic and would make a bad Shadow.
Hearing Deven Mack in Sonic Prime was a big relief. His voice is very goofy and lively, yet gentle and soft, I think it was perfect, it portrays Sonic's personality and emotions, is versatile and I hoped for him to became the new Sonic voice. Unfortunately we got RCS back.
Fan VA
As for fan VA , the first that comes to my mind is Pixelrush. I was surprised that a goofy voice would sound so good on Sonic.
LittleGameBoy5 sounds very dark. I can't say I hate his voice. He dubs the gloomy Sonic from Idol Amy, so it suits. He is also capable to be goofy when needed.
PatchToons, (Sonic and Amy Squad) dubs all male Sonic Characters but Tails. And Mario who is the guest, making all of them sound recognizable as their own. For Sonic, it seems to me he is referring on RCS, because the voice he uses is quite low.
Shadow - The officials
I've read more than once that Shadow's voice is supposed to sound beautiful.
Anyway, My favourite VA for Shadow is David Humphrey, and his interpretation is fairly different than the others that followed. He dubbed Shadow in his debut game and in Heroes. I think the first thing that I noticed is the low volume of his voice. This underlines a thoghful personality. He also spoke very clear. The voice sounded youthful and smooth and the range of emotion was large, Shadow was a grieving kid, but he was also mischievous, and you could clearly hear that. David sounded so good because that was his natural voice.
I also like Jason Griffith. He's not David and he makes Shadow sounds too mature at times, but he clearly captures his melancholy. Like for Sonic, he may sound cute. Many complain about Jason's voice being wispering/breathy when he does Shadow, but I like that. I think it underlines Shadow's quietness. Although I know some taciturn people who are loud talkers, usually when you aren't use to talk, your voice loses strenght.
Jason's Shadow can also sound playful. In Sonic Zero gravity Shadow is having fun and you can hear that.
There is not much from Ryan Drummond as Shadow. He had to imitate David, so I don't know what his take would have been, but unlike many I think he didn't sound like Kirk.
Now is better to restrict myself because what I think about Kirk Thornton's voice is... how to say? Kirk's voice is... uhmm, not very beautiful. I don't complain about the pitch but I can't stand the raspiness, he makes Shadow sounds always angry and murderous even when he is not supposed to sound like that. The famous 'You got this Sonic!' sounds angry when it shouldn't have had. The voicelines I heard on YT were at best embarrassing.
In first episodes of Sonic Prime, Ian Hanlin didn't made a good impression. He was so loud. But when Shadow had to talk for long (longer than he ever talked) in the episode 9 and when he appeared in the following episodes, I coul hear the voice I was used to, soft spoken and soothing. He shouted in the first episode for a good reason.
Fan VA
The first that I think about is Crazy Greg. He dubs in Idol Amy and has his own Youtube channel and is active for a while. Clearly he bases his dubbing style on Jason Griffith, but his voice is very iconic, powerful and feels so Shadow-like, many think SEGA should hire him.
Another VA I like has a unequivocal username... Shadow the Hedgehog, known also as Julz. He is normally a loud talker and has a goofy voice. I was surprised when he started to write serious stories with Shadow as protagonist, played them in cinematic VRchat and dubbed him... his voice suddenly sounded so soft and sad... and still recognizable as his own.
SimitarchVOs is interesting... his voice is quiet but also raspy and deep, certainly not beautiful... and yet is charming and fun, a bit sad, it sounds like somebody who went through a lot of tragedies in his life and is now trying to find peace. He mostly dubs with Pixelrush.
Shadow759 and Vindo (Both dub for Sasso Studio, but Shadow759 has his own page with his own videos) sound natural. They just speak calmly but they don-t go too far from theri natural voices, a bit like David Humphrey. This make them sound really smooth. Shadow759 also makes a good Silver. He is alittle less convincing as Sonic though.
Last, PatchToons from Sonic and Amy Squad, with his soothing, quiet and slightly breathy interpretation of Shadow. Interestingly, for Sonic he refers to RCS. But for Shadow he makes him sounds like a mix of David and Jason.
There are so many fan VA around, both good and bad. I choose to talk about those that i like most, but there are so many good fan VA around. Too many to be mentioned.
There is no point on talking about fan VA I don't like.