which part of great wall is better to visit and also easier to go?

Hi.I will be in beijin at the end of this week and I will be in beijing just for 3 days and I have to go to exhibition too.and I want to visit greatwall and world park.could you please tell me which part of great wall is better to visit and also easier to go?and could you please tell me which places is more better for me to visit because I don't have enough time for visiting all sightseeing places.thanks in advance.