Having to just pick one soundtrack from that series is nearly impossible. However, if I had to pick just one, I'd have to say Sonic Adventure 2's soundtrack because I feel like that game had some of the catchiest and memorable music I've ever heard in any video game. Some other games in the series that have incredible soundtracks are Sonic Colors, Sonic 2, Sonic 3 & Knuckles, and Sonic Adventure. There are plenty of other games in the series that have amazing soundtracks, but those ones I mentioned stick out the most in my opinion.

GenesisJunkie : My favourite music has to be from Sonic Unleased for the Wii, that game rocks hard in the
music department, I wish I could say the same for the gameplay...

In my opinion Sonic Generations had the best music since it took all the best tracks from the previous games and made them either 20 times more better or added an interesting spin to them. 

I think Sonic Unleashed and Lost World had good music because of the orchestra doing the music, but not my favorite, as the title still goes to Sonic 06.

I'll just go out on a limb and say Sonic 3 & Knuckles and Sonic Adventure 2.
Pumpkin Hill, Live & Learn, and E.G.G.M.A.N. are my jams!

Honestly I have to say sonic unleashed. Not only because it was my favorite game, but because the music was so distinct felling native to the country you were in

For the most part, there are many many sonic soundtracks that I like, but I think my favourite might just be Sonic Heroes. Sure, Generations had a stellar soundtrack, but I'm really not a huge fan of the Seaside Hill themes or Green Hill (Otherwise, this would be my favourite, hands down).

If we are talking classic 2D Sonic, my favorite personally was Sonic 3.  I loved how funky and catchy some of the tunes were.  My favorite track from it is still Hydrocity Act 1.  As far as modern 3D Sonic games, probably Sonic Adventure 2.  Songs like Escape From The City, Live and Learn, and Believe In Myself are some of my favorite tracks in a Sonic game.

A lot of the sonic games have outstanding music even Sonic R.

Sonic Adventure, Sonic Adventure 2, Sonic 2, Sonic Colors, Sonic Generation's..etc

Heck even Sonic 06 has some pretty good music. But if I really had to choose just one I would have to go with Sonic Adventure. There really is a lot of good tracks to listen to in this game my favorites being red mountain, lost world and windy valley. If I could find a physical copy of the soundtrack I would buy it in a heart beat. Sonic Generations is a close second with Sonic Unleashed following right behind it.
If anyone wanted to listen to some sound tracks from games, music from the Sonic franchise is definitely the way to go.

" WHOO! Feeling Good " - Sonic [Sonic Unleashed]

The initial trilogy has some of the best soundtracks of the 90s. There are plenty of great tunes in all of them.

However, my vote for the best Sonic soundtrack would have to be Adventure. Plenty of great memories playing that on the Dreamcast and enjoying the sounds.

Honestly in my opinion Sonic Adventure (any version). It has a pretty good soundtrack and also has Open Your Heart by Crush 40. I mean just check out the whole soundtrack and maybe you'd think it's pretty cool!

Image upload: 225x225 totaling 9 KB's.

Probably not a lot of people say this, but I say sonic colors. The music really fit the game itself and my definite favorite is reach for the stars because it screamed colorful and sonic in it .(not literally if you haven't heard it )

To me the sonic game that has the best music is the first sonic ever made. I loved the original music and I wish they kept it through out the entire game

For me and my brother we would say the sonic games with the best soundtrack would be sonic adventure 2 battle or sonic generations

Sonic 2 had some of the more classic sounding tracks that are synonymous with the franchise itself. The first Sonic has some good songs as well, but the music (and the game play itself) became more polished by the second installment and it definitely has more of the retro classic songs that I will always remember.

Interesting that OP mentioned Sonic CD though. That and Sonic Adventure 2, for me, has the most memorable of soundtracks because they ARE complete songs and given that the original soundtrack was written by a J-Pop artist, he likely had more full, complete songs in mind but due to the limitations of the time, he couldn't. Sonic Adventure has songs that fit in perfectly with the contemporary music of the time and made it paced well enough to make the levels feel like they really were an Adventure.

Sonic CD definitely has the most memorable soundtrack because of the atmosphere of the tracks. Rather than faster-paced songs that would fit in with the fast paced game play of normal Sonic games, Sonic CD's ambient smooth tracks force you to pay attention to the bigger picture, the level design, and the overall artistic direction and structure of the game. The quiet, sultry female voices lather the track with a hypnotic quality about them and as OP said, they are tracks that make you feel like you're going on a long road trip into unknown territory-much like Sonic CD itself.

The classic sonic game had some pretty memorable music, so I'll go with that.

To this day, I am still voting for Sonic 3 and Knuckles. It was not only Sonic's best adventure, but also the one with the best soundtrack. All act 2 were a remix of act 1, which was cleverly done. Carnival Night is almost indistinguishable! My favorite ones are Angel Island 1, Carnival Night and Death Egg 2.

I also like the Special Stage. Fun story: I used to record the music my putting my old red tape recorder next to the TV. However in the beginning I was not very apt for it so I often missed the first few seconds. I was frustrating for the Special Stage since the intro was a lot of fun to listen to. So when I first downloaded the MP3 (and even today) I would just repeat the intro over and over again. Too bad that the CD soundtrack does not accelerate the special stage as it does in-game when you spend too much time.

For as absolutely terrible as the game is, Sonic 06. Sure not every track is memorable, but it's got some really great tracks. Wave Ocean being my personal favorite track in the series.

As much as I hate Sonic 06, I do absolutely love the soundtrack. Dreams of an Absolution is one of the best songs I have ever heard, His World is great, and the stage music is nice for the background. I just wish the game was good so I could play these songs while I am playing the game.