The lower the gear (higher number) the harder its going to launch. but it will also rev out quicker. If you can maintain traction at launch that'd be good.
The object is to keep the car within its power range. You need to know what your rpm power range is. You may be reaching the limit too slowly or too quickly and changing gear too late. You want to be launching with enough revs to have the car within its power range immediately.
Try these online calculators. The ones down the bottom help with tire size, gear ratio etc. Should help.
http://www.golenengineservice.com/html/calculators.html