Genuinely, I dont think that many firms have a ranking system. Firms are always interested to look at extra curricularactivities not because of what they have involved specifically, but more in relation to what those activities might say about the candidate. For instance, being involved in the Duke of Edinburgh Scheme is not in itself of much interest to an interviewer but being aware that a candidate might have planned and led a trip across Dartmoor would, because it shows organisational qualities, leadership skills and the ability to deal with challenging situations. So, think about activities which help to demonstrate that you have the sort of qualities that a law firm is looking for i.e. leadership, initiative, drive and the ability to cope under pressure.