Just to clarify, medal and equitation classes are judged on the rider only. Hunters are judged on the horse, their way of going, and style of jumping. However, even though only the rider is judged in a medal class, you need a good horse to help you perform everything they ask of you!
Personally, I would start in the hunter or equitation ring, just because the basic skills that you will need are stressed here. The ability to make nice, smooth turns, keeping an even pace, finding a good distance, and- in equitation- your riding/position overall. Because jumpers are based on time, it can be easy to forget about the quality of your ride in the beginning and just focus on speed, which isn't what you want right now.
I think it's important to try everything out eventually, though, because it makes you well-rounded as a rider to be competent at each.