Someone can't really tell you which is more fun, I honestly have no idea which role i like most(even support) Just play each role in draft to figure out which YOU like. Just to make it easier though:
Mid has lots of control over the game. They can roam to top or bottom lane and have the most resources with Blue Buff, wraiths, and wolves near. Mid tends to be more action packed early than ADC. There is a lot for AP to do in laning.
ADC tends to be 15 minutes of farm fest, occasionally getting ganks, or going for dragon. There is also a lot of pressure on AD later in the game as they do have to most potential to do tonnes of damage. If you are a passive player in laning phase, and prefer to farm rather than fight your opponent, you might like AD. There is also the issue of synergizing with a support. When I played AD for my team, my support and I just didn't work well together, and it wasn't optimal.(Oddly, when we switched roles we did well) Just keep that in mind, you need to be able to work with your support.