It depends, but IMO just about any workout you find will be effective, for about six weeks. After that your body adapts, or in my case goes into overtraining syndrome, and gains stop.
For muscle building, look for a workout that focuses on heavy compound movements: Squats, deadlifts, bench press, etc. But, don't expect to see a lot of gains in only five months. Depending on your body composition at the moment, you may want to focus more on toning the muscle you already have and stripping off as much fat as possible. Most bodybuilders spend between four and five months prepping for contests. The increased muscularity will give the illusion of more size and make you look awesome in your costume!