I've always seen soccer as strictly a cardio thing. That and flexibility. I guess you could call it difficult because it requires a ton of cardio training. However, I actually hear boxing quite often because to be a good boxer, you have to be quick, agile, powerful, sharp, and flexible.
I've played Baseball, Ice/Roller Hockey, American Football, and even some recreational Soccer way way back in the day. With all my knowledge playing these sports... There is still one that absolutely takes the cake in terms of pure requirement of peak fitness, and that is Wrestling.
Its one of those sports where your core fitness is needed more than any other sport out there. I've always been in tiptop shape while I was playing my sports... and yet I never came near as fit as the guys I knew who Wrestled Competitively.