Sooo, Sonic Free riders. Dear lord, I never disliked Sonic and his friends as much as here.
I will never say that any one Sonic game or plot will "Ruin the series forever!!" since every Sonic game clearly has it's own style. But Free Riders is the ultimate example of how bad the series can get in it's most extreme cases.
(as for as the story goes that is. Can't judge the gameplay since I never understood the appeal of the flying boards, so I leave that to judge for the experts)

Sonic is practically a Gary Stue character. He's awesome, he's perfect, better then everyone else. None of the other characters has a purpose.
Now, in the 2d games, this wasn't a problem, it's just your awesome hedgehog with an attitude fighting robots.
In Sonic adventure uno and duo, they did a decent job writing the story around it, and gave the supporting characters each their own little story arc, each with their own motivations, own lessons and morals to be learned and each with their own little victories.
However, ever since Sonic heroes this hasn't been the case much. Sure, the supporting cast still get's their own storylines, but they're more...well..excuses.
Characters like Shadow and Amy and all get one OBSESSION, their reason why to rush after Sonic/Eggman (The lucky supporting cast that is, some (knuckles in particular) doesn't even have that luxury) They don't have their own arc, they don't learn any lessons, they don't succeed in any goal, they do nothing. They just rush after Sonic, every dialog being nothing more then CONSTANTLY REPEATING WHAT THEY'RE OBSESSED WITH, and when they finally catch up with the main plot, they're immediately shoved to the sidelines, either begging sonic to help them or just cheering him on.
In the end, it's all about Sonic.

What does this have to do with Free Riders? Every single flaw that the previous Sonic games and characters might have had, is ramped up to level 100. There's no plot in this game, nothing to distract you from from one pure, dark, bitter, violent and horrifying look into the characters.
The plot (what little there is) is all about Sonic. Sure, there's some treasure mentioned, and Vector and Wave are interested in it, maybe Team Dark too. But that's mostly played for laughs. No, the real story is that every single character is desperately trying to beat Sonic, to earn Sonic's attention and admiration. Amy's obsessed with Sonic (well, more then usual) Jet's obsessed with Sonic. Eggman's entire plan ends up him trying to impress Sonic and Metal Sonic's secret hidden agenda ends up him trying to impress Sonic.
And every dialog you get is nothing but characters endlessly repeating, endlessly screaming about their obsessions. Other then Team Dark, Cream, and maybe Tails and Wave, you'll gonna hate every one of them.