Who Should I Start Liberty On?

We can't say. But try with the one most calm around treats. I made the mistake to think the extremely food motivated would be easy and got a looong time of working through aggression and anxiety with her (aggression was due to anxiety, but I didn't realize at first) and let me tell you, it HURT to train with a pony like that when you don't know what you're doing. So yeah. I would start with one that already have decent treat manners to practise teaching them without it getting dangerous, just until you've learned how to teach them to be safe around treats. That super anxious one i had have never beahved better than now.