Football.
It really all depends on the look you want. Swimming, long distance running, soccer, and tennis athletes all have a very lean physique that's geared more towards cardio and efficiency. They won't be very built, muscle wise.
Football, rugby, and wrestling athletes have much more muscle on their body. They look "jacked".
Boxing is too broad a sport to really put in a category. Lightweight boxers are built more like swimmers because it's highly aerobic, and heavyweight boxers are built more like football players. They throw less punches and move around much less. Also, pro boxing matches can go for 8, 10, or even 12 rounds, while amateur boxing usually lasts only 3 rounds. Because of this, you'll usually see much bigger amateur heavyweight boxers than pro heavyweight boxers, since you need less than half the endurance. Try sticking an amateur boxer in a pro match and he'll be winded by the 4th round, since it would be a waste to have less muscle/more endurance in a 3 round fight. Imagine a long distance runner competing in a 100 yard dash. The lack of muscle and aerobic conditioning helps them run much longer, but they lose explosiveness.