The best is obviously the trek itself ! You walk for 4 days in tough conditions but it's clearly worth it. The highlights are the second and third days which are the toughest ones as you feel that you push your limits a little.
I would have hated Machu Picchu without the trek as it is well too touristy ! With the trek, it all made sense which was great. I was not a trekker before but I am clearly keen to do more of them now ! Let's see where our next adventures take us. Who knows where it will be now ?