i'll only be talking about the mainline games, because everybody already knows that games like sonic shuffle and sonic labyrinth are abysmal.

sonic 1 - the game starts strong with green hill zone, which allows you to Go Fast™ along multiple paths with an expansive three levels. then marble hill zone starts. you are funneled down a single path while you complete "puzzles" such as "pushing a block" and "standing still on a block" that do nothing but slow the game down. why would you set to slow the player down in a game about speed? who knows. the game is *okay* but flawed. certainly not the "mario killer".
sonic 2 - sega proves that marble hill zone was on purpose with chemical plant zone, which mixes underwater levels (breath timer included) with time-based platforming. the game is much of the same, but this time, tails is included. there is a shitty multiplayer mode.
sonic CD - introduces a novel time travel mechanic and doesn't explain it. you can beat the game without ever using it. none of the levels are ever shitty, merely mediocre. widely considered the best sonic game.
sonic chaos - all the fun of the originals on much weaker hardware. you get to play as everyone's favourite character tails.
sonic 3 - again, much of the same. the "& knuckles" thing was a great idea but ultately little more than a gimmick that added a slow character to a speed-based game. the time travel from CD is gone, and will never appear again in any other sonic game.
knuckles chaotix - an okay game released exclusively for a shitty peripheral for the dying sega genesis.
sonic adventure - many franchises died when they tried to move to 3D - gex, bubsy - others released terrible 3D games and never tried it again - megaman, castlevania - and others executed it perfectly - mario, zelda. so where does adventure fall on this list? i wish i could say the first group, but the truth is that despite fucking it up, the franchise survived, continues to survive, and they keep making terrible 3D games. horrible voice acting, a hostile camera, forgettable locations, a ridiculous story, laughable boss fights (oh no.), and the introduction of the homing attack (more on that later) make this a real stinker. also, once you beat it as sonic, there are five more characters to play as. big the cat's gameplay is based on a fucking fishing minigame, knuckles' story consists of running around searching for gems with a metal detector, and the others aren't worth talking about. the dreamcast's best selling game.
sonic adventure 2 - thankfully, there's no fishing game this time. rather than beating the game multiple times, you simply switch between multiple characters, which is a much better approach, because you get to play less sonic adventure 2. introduces shadow the edgehog and rouge the bat, in case you thought amy the hedgehog wasn't stereotypical enough of a female character.
sonic heroes - do you like playing 40 minute long levels with no save points? the horrifyingly ba lip sync/voice acting from the previous games is gone, apart from a few examples (look at all those eggman's robots!). this is the point where it starts becoming obvious why the homing attack is a bad idea: all of the combat in the game consists of pressing the A button over and over again. A to jump, A to attack, repeat. you can switch between the speed, strength, and flying characters, and the levels "cleverly" force you to do so at certain points. there are four teams to play as, with team rose being the easiest, then team sonic, then team dark. team chaotix changes the game from a fairly mediocre 3D game by sonic standards to a boring, shitty collectathon where you scour the levels looking for tiny objects.
shadow the hedgehog - i don't know if this one's really a mainline game, but whatever. the shallow combat of heroes is deepened slightly with a """morality system""" and guns. yes, this is a sonic series game where you use guns. there's a reason people call him "shadow the edgehog". the game sucks.
sonic the hedgehog (2006): do i really need to say it? even the sonic twitter account acknowledges that this one was horrible.
sonic and the secret rings - it's a wii exclusive, and you know what that means: shitty motion controls! to quote the sonic wiki: "Unlike other Sonic games, in which Sonic is controlled directly, Secret Rings features an indirect "on-rails" method of control in which Sonic moves forward automatically in a semi-linear path. Sonic's movement can be controlled by tilting the controller left or right.". remember rails shooters? remember playing those at the arcades in the early 2000's? weren't those fun? no? yeah.
sonic unleashed - it's actually a pretty fun 3D sonic game. it has one major flaw, however: the mechanic the game was named after. when you transform into a werehog (actual terminology), the game turns into a crappy beat-em-up that chugs along with horrible framerates.
sonic and the black knight: a wii-exclusive sequel to secret rings. fill in the blanks.
sonic 4: split into two episodes because money. a bad game capitalising on nostalgia for a bad game, with broken physics and the return of the beloved homing attack. at least the jumping feels a little better than the genesis games.
sonic colors: to quote the wiki, "Sonic Team examined criticism of previous Sonic titles from critics and fans and tailored the game to match. The resulting changes included an amalgamation of 2.5D and 3D level designs and graphics, and omission of a core "gimmick" such as the sword in Sonic and the Black Knight." the sonic team knows they their games suck. unfortunately, it's still a sonic game, and many of the flaws still appear here. overall, it's one of my favourites. the wii version and DS version are completely different, and both are pretty good, but not amazing.
sonic generations: a combination of the "fun" 2D games and the "fun" 3D sections from unleashed's non-warehog areas. not bad, but not great either. hilariously includes a scene where sonic tells his past self "your future's gonna be great!"
sonic: lost world - just play super mario galaxy. the 3D sections suck and seem maliciously design (invisible springs? what the fuck?) and the 2D is just as mediocre as ever. to quote empire's review, "in overtly coveting the great Italian plumber, it smothers the talents of its blazing blue hedgehog."
the sonic boom games: rushed. broken. monotonous. far too easy.
sonic mania: remember the first three games? it's those, but better. the new levels are much better than the remixed old ones. i stopped playing at flying battery zone because the hostile level design became too much for me.
sonic forces: the gimmick of this game is the character creator. everything else is utterly unremarkable.

what a fantastic series