SONIC UNLEASHED

To get this you have to finish an Act with Sonic and get an S ranking. I found this easier in the beginning levels (Windmill Isle Act 2 is recommended).

Make sure you learn the layout of a level so you know when to jump/slide/boost etc... Also wherever possible hold the X button down to gain massive speed boosts (don't just tap it like most people do).

Videos below of my exploits so far. Choose what you think is easiest!
Hope you enjoy. As bad as it sounds, it's a good idea to restart if you die--adding up points from the beginning instead of a recent checkpoint will--obviously--net you a higher score.

I unlocked this one while also going for the Blue Meteor Achievement. Since you lose so much time when you die, you really need a flawless run to get Blue Meteor, and the lack of death means all your points add up and will be multiplied by your fast time, making an S Rank easy to obtain.

There's only a handful of parts to Windmill Isle Act 1 that can slow you down, the low sandbag wall at the beginning if you don't jump, the walls you have to slide down, and after the long bit where you're grinding on the rails the first time there's a jump that lets you skip a brief platforming section. It's a short section...but every second counts. I messed up a jump at the very end of the level but because I had maxed out my speed and ring energy gauges I still made it to the end before the time required for Blue Meteor. Practice the level a few times to get a feel for it BEFORE you try to S Rank it. You need to practice it at normal speed before trying to run the level at...however fast Sonic can run with a maxed out speed gauge.

I find it easier to get S rank on Act 2 since they only take 1 minutes to beat most of the time. Remember, it'll be hard to get an S rank by getting there with good times, you have to kill enemies and get rings (you can do both by Boasting towards them)