i have just purchased a sidewinder for my sc10, i went with this esc cuz it's cheap and i only plan on running in a stock SC class that requires a RTR motor, 27t brushed, or a 17.5t brushless.
my question is what is the best 17.5 brushless motor that can be used with this esc without major cogging issues?????

While I can't give you advice on a motor you definately want to pick up the Castle Link and load the latest firmware into the ESC. I had cogging issues with my Sidewinder 5700 combo and discovered that the ESC firmware was an early revision. They released firmware in August to deal with some issues and it was a marked improvement. It's $25 well spent, you can make adjustments much easier than through the TX while others can be customized only through the link.

They will all cogg roughly the same with any 17.5. The Sidewinder was not made to run sensored motors. If you want a Castle Speedo that can do that you need the new Mamba Max Pro which is going to sell for around 130 bucks. It is Castles new speedo that is made to run sensored or sensorless motor and has a port to plug the sensor cord in.