Ableton, but I use it like a spastic. I haven't learnt any of the beat session stuff I see on yootoob tutorial vids, no MPC or Maschine, it's all manual chops, I don't even use any of ableton's groove settings when I warp a sample - all straight and listen for where I think it needs to come in a little early or late. Which is time consuming as fuck.
If you don't have Ableton you can make a basic loop in audacity too, it's simple, I was explaining to a friend who wanted me to make him a drum loop from a sample, find an electronic programmed track at a similar tempo and paste it into your audacity, then cut and paste all of your kicks and snares underneath it, and you've got a template rhythm that's regular and in time. With ableton you've got as many extra channels as you want to build on it, not sure if you can save two tracks (in effect channels) in audacity at the same time though.