I would either go avionics or mechanical, they will open far more doors post service career, as said it depends really if you see yourself as more electronically minded than mechanically, that said a lot of the mechanical trade now contains electrical and electronics. Civilian wise avionics trades tend to be at a premium these days. I am an ex RAF mechanical engineer who studied and put myself through my licences, (start them in the services if you see your future out of the RAF continuing your future trade) but to be honest the service engineering background will stand you in good stead whatever you do.
The other one on that list to look at is the ICT Technician as that to crosses over to a well paid civilian job.