I personally liked these two combos. These low attacks give a surprising amount of range, but are easily punished.
Down + Y, Y, Down + A
Down + Y, Y, Jump + Y, X, X
[enhanced] Down + Y, Y, Jump + Y, Down + A
The fact that you can combo out of midair X is actually pretty great and gives machamp a little bit of variety, especially against an opponent who likes to rush in.
I have to sit down and practice this, but you can launch and juggle twice with this. I just need to go into practice mode and see what I can finish with.
Down + Y, Y, Jump + Y, Down + Y, Y ....
I'm not good at fighting games that aren't named Smash, so if the combos are easy for me to remember and use, they're probably worth looking at.