@Background Pony  
Also, 2700 mph is fast, yes, but Sonic has to traverse terrain as well, you would need the reaction time of a supercomputer on crack to traverse around the landscapes of the sonic series at half that speed without becoming Sonic the Splat-On-The-Wall.

You my friend are the smartest guy I know on this site. And from both of our points of view, I guess we can both say that Sonic is indeed faster than Rainbow Dash. He’s the fastest thing alive. And that’s a proven fact.

@Background Pony  
My point is that there are several occasions of the game’s information pretty much TELLING us that his speed is hypersonic. Hell, there’s a point in Colours DS where, if you S-rank a mission, Omega makes a comment about Sonic ‘approaching light speed’. Mind you, this could just as easily be due to that stage utilizing Laser/Cyan Wisps, but again, it’s all up in the air and unconfirmed.
 
I don’t really take the Game Theory thing into consideration either because it falls into various degrees of misinformation and hyperbole (for example: the guy making such a big deal about ‘OMG I PLAYED SONIC ‘06 TO GET THE HEIGHT INFOz!!’ when Sonic’s height and weight information is easily accessible on Sega’s official Sonic Channel site). Plus it neglects a key attribute as to Sonic’s speed: His personality. I’ve said it before, Sonic’s a show-off. He’s arrogant, cocky, and full of himself. At no point does he ever feel the need to truly ‘push’ himself to his utmost limits in order to save the day. It’s like The Flash from DC Comics. Dude can move so fast he can vibrate back-and-forth through time and travel faster than light. Does he do this ALL the time though? No. Why? He doesn’t NEED to. He’s the fastest thing alive, after all. He can do things at his own pace. Which is the same as what Sonic does.
 
If you’re basing this off of the information that the games give us, then you also need to take the fact that Sonic’s personality also affects his performance, because that is also information that the games give us. Sonic’d rather dick around and be a jerk, and toy with his opponent, rather than just go ‘MAXIMUM SPEED! RAPERAPERAPERAPERAPE!’ on an enemy. I mean, hell, when you think about the boss battles with Eggman, either one of two things applies here: Either Sonic’s intentionally slowing down just so he can ‘play’ with Eggman, or Eggman is officially THE most unbeatable villain in history because all of his mechs are moving at hypersonic speeds all the time.
 
All we know is that Sonic’s speed is in the ‘hypersonic’ territories. That means anywhere from Mach 5 (3,840 mph) and up to Mach 10 (7,680 mph). We don’t really have any other specifications beyond those, at the end of the day, but based on that much alone we can definitely assume he can match-or-outmatch a Sonic Rainboom.

@Background Pony  
I’m not defining Sonic by the limits of the game, I’m defining Sonic by the limits that the information of the game gives us. Remember the Game Theory episode about Sonic? Just because Sega says Sonic goes at a certain speed doesn’t mean that he’ll look like that as he would in real life. And about Sonic Battle, fighting and racing are two different things. And about your point about Sonic Colors, Sonic was able to outrun that black hole until it caught up to him because he wasn’t in the event horizon, so any speed would be enough to get away from it.
 
I will admit I was wrong about the 2700 mph part, but I also said it was “around” that. It’s actually about 2900 mph on second look at game footage.
 
And since this isn’t real life, you can’t say Sonic is ever going to get tired if he reaches his maximum speed. Rainbow Dash didn’t look particularly exhausted after she achieves sonic speed, even though we know that it’s her top speed for sure.

@Background Pony  
You can reach 2700+ without boosting in Unleashed. It’s called exploiting the momentum physics with drifting and quick-jumping. In fact, you can easily exceed 2700 using this in combination with the Boost, but doing so causes you to go so fast that the stage lags when it comes to loading (see: A few of Kakipi’s speedruns, as well as plenty of other pro speedruns).
 
So again, the 2700 marker is just the enforced limit that they put on the game because the levels can’t load that quickly in comparison to the rate of progression through the stage. It’s just the same as how in Sonic Adventure there’s an enforced limit to your acceleration and your max speed in the engine. With a modding device, you can actually increase the rate of acceleration AND the max speed to reach indefinite levels - but doing so makes it impossible to properly control Sonic due to him going too fast.
 
So again, defining his top speed by the limitations of the GAME isn’t really a good idea. Especially when there’s one other factor to consider here: Sonic fools around. What do I mean by this? I mean that Sonic, rarely, if ever, goes to his max capacity. Have you ever seen a moment where he was out of breath or looked like he was particularly taxed by the speeds he was moving at? No. Why? Because he wasn’t even trying yet.
 
2,700mph for Sonic would constitute a ‘light jog’ since he can keep it up pretty much indefinitely, and doesn’t even break a sweat in doing so. Contrast this with two other games where Sonic’s clearly moving WAY faster than a mere 2,700mph.
 
In Sonic Battle, Emerl has absorbed Sonic’s speed (or more accurately, the speed that Sonic had displayed to him at the time), along with the powers and abilities of the entire rest of the cast, AND is hopped up on the power of all SEVEN Emeralds, and is about to wipe out the planet unless Sonic can beat him in THIRTY SECONDS OR LESS. Which he then does. So not only can Sonic exceed the speed he normally shows people, but he can do it against an enemy who is already enhancing said speed.
 
Following up with this we have Sonic Colours. Y’know. The one where he outruns a black hole for all of thirty seconds? Lil’ bit faster than 2,700mph on that front.

@Background Pony  
I assume that about 2700 is Sonic’s max speed because he needs the Sonic Boost to go that fast. And because Boost energy is limited, the boost is a sprint that Sonic can’t hold forever. The speed of sound is about 760 mph, so the Unleashed speed is already several times the speed of sound.

@Xyres  
I’m a nerd.. it’s in my nature to be number-crunchy about trivial facts about fictional characters. >_>

@Background Pony  
Yoooo, someone that knows his shit right here.

@Background Pony  
‘Cept it’s not his max speed, because that’s just the fastest he can go in Unleashed. That’s neither a sign of his ACTUAL maximum speed, or is it a declaration of his maximum speed either. The Sonic Adventure manual states Sonic’s speed is ‘hypersonic’, and Sonic Battle’s makes mentions that his speed ‘moves at multiple times the speed of sound’, making his speed, again, an unconfirmed number.
 
And the Mach 5.4 thing comes from the Physics is Magic video, where someone took the time to calculate the visual angle and shift and trajectory of the Sonic Rainboom and calculated the velocity thereof.
 
So yeah. Sonic’s NOT limited to just 2,700mph. That’s just the fastest the Unleashed Engine allows you to go without, y’know, breaking the freaking game and destroying the physics.

Why the hell do people say that the Mach 5.4 is Rainbow’s maximum speed? Unless it comes from Hasbro or Studio B themselves, I won’t buy any of it.
 
At least we know Sonic’s max speed is about 2700 mph, which is from an official source (Unleashed).

Sonic ‘Boost’ is just a game-based representation of Sonic punching the sound barrier in the sack with enough force to cause a cone to form.
 
Since Sonic as a whole has no known ‘top speed’ outside of simply being dubbed ‘hypersonic’ (i.e. anywhere from Mach 5 to Mach 10), and the Sonic Rainboom is confirmed by fans who did the math to clock a maximum of Mach 5.4 in terms of speed..
 
..no contest here, Sonic Boost beats Sonic Rainboom, just because that alone implies that Boost could easily have him outdoing Dash’s maximum speed without having even begun to reach his own maximum velocity.

@Background Pony  
A tie? BITCH PLEASE
 
http://www.youtube.com/watch?v=Glfx6LBbbGY
 
KAKIPI1000 proved that RD is slower than Sonic.

@williamcll
 
Stealthy? The boost creates a boom, too. How is that subtle in the least?

Also, Sonic boost is a lot more stealthier, much easier to sneak around evil robots.

It’s a tie.
 
Rainboom is a lot flashier and it’s explosion is considerably powerful, but tou can do a very painfull face-slam if you’re not carefull.
 
Boost is nowhere near as epic-looking, but it sends you flying everywhere like an insane hypersonic cannonball of doom and plow through everything.