My criteria if I have to fly Economy is "which flight gives me best chance of empty seat next to me?" I can live with legroom, but am a little on the broad side. In that respect, I'd be looking for a 747, pref Low J (= more Y seats), and would choose an aisle seat towards the back in the middle block of 4. Food and entertainment (unless you can guarantee new AVOD with 773 or 787) shouldn't vary too much. Another line I take is to look which flights show Avios avavilabilty on WT+ as those are more likely to generate cost-effective POUG offers to W.