It really depends on the sport and position - football (American? European?) and rugby are two different sports that require very different motions and strengths.
For rugby though, you need overall upper and lower body strength, but I feel that your core and legs are most important. Take a look at most (or all) rugby players' quads - they're pretty diesel. If you're a pack player, your lower body and core will help you drive and push in scrums and rucks.
But the most important thing about rugby is endurance - bulk is not very helpful without the ability to tackle, be tackled, and repeat for 80 minutes without stopping.
A combination of sprinting, lunges, ab exercises, wall sits are a good circuit workout.