Mine would be Sonic Adventure. It's got a good story to it, and good gameplay (except when you're playing Big, it can get annoying trying to catch Froggy), and an awesome soundtrack. Also it's where Eggman first gets his nickname
Now you might say it's sacrilegious to not say one of the first four Sonic games was my favorite, but I didn't get into the Sonic series until between Sonic Colors and Sonic Generations. While Shadow the Hedgehog might be my second favorite game, Sonic the Hedgehog 2 is my third.
Sonic the Hedgehog 3, Sonic & Knuckles, Sonic Rush, and Sonic Generations aren't too bad either. I want to say I liked Sonic the Hedgehog 1, but it was just too hard I wouldn't have beaten if it I didn't play it on DS where you can save at the beginning of each level, took forever to get past some of those levels and had to check a walkthrough for the final level. Ironically enough the final boss was easy
And here's another thing: Is it true that some people hate Tails because he's the reason they keep introducing new characters? Do you? Be honest, I am a member of a certain other fandom all of whose members respect the opinions of others. Is it true that you hate the addition of new characters, and think that Sonic and Eggman, or maybe Sonic, Tails, Knuckles, Eggman, Amy, and Metal Sonic, should be the only characters? Honestly I'm a little disappointed where we don't see some of the other characters, particularly Shadow, Espio, Vector, and Blaze. (earlier games like Sonic the Hedgehog 2 don't count for disappointment coz this was before they were introduced) The reason I'm so defensive of Tails is he's my favorite character, and one of my favorite furries