That's what i love so much about Sonic games. They always have an interesting storyline for players to know the ending, even if the gameplay is a little... you know....
Sonic started not being as serious, with Sonic freeing his animal friends from their robot captors, but i just love how it went to that to... well here's just a small list of what awesome storylines.
(I won't try to spoil anything, but if i do i am sorry)
1. Sonic Adventure had the whole tragic past going with Tikal and the echidnas. Not to mention Gamma's story being the most thought-out storyline
2. Sonic Adventure 2 had more tragic past only in the form of Shadow and his past. (Did i mention that this game had some of the best music? I love "Supporting Me" the song while fighting Biolizard). Basically the serious plot is based all around on Shadow, and also what could have easily turned into something like 24 with the "president must give in to evil eggman's demands or else he will blow up his part of the world".
3. Sonic Heroes might have been the norm 3D and some cheesy dialouge (If you remember "We're Sonic Heroes!"
![Urg :urg: :urg:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
) But the whole return of Metal Sonic was sweet to hardcore Sonic fans. I think the only MAIN problem in that game is that everybody had the same exact levels and bosses.
4. Shadow the Hedgehog probably is either the most serious or 2nd most storyline (depending on if you think Sonic 2006 had a more serious storyline) Shadow had lots of plotholes, which got away from the flow of usual games. But Shadow could become ruler of the world, universe, or the main lackey of the evil black arms. (Oh and an attempt to kill the president in one mission)