Geez Tom, as a hard question why don't you.
Of course the answers you will get to this will be so varied it will make interesting reading as people argue their various points as each person will have some aspect that they add more 'weight' to than others and so on.
For me, I will start by saying that we are talking riding/racing and not the off track type of activities (good or bad) and of course the 'most talented' does not in a lot of cases mean the best or rider with best results.
I will start by naming the most talented rider I have seen as Anthony Gobert who as we know has problems off the track, but on it was pure dynamite.
When I see talent it is about the bike control and how a rider reacts under a number of different circumstances that makes me think that this rider is talented. It is how they pass a backmarker who does not know they are there, or how they catch and pass for position following a bad start or mishap. How do they react to another rider's error affecting them, do they lose focus and ride like an accident waiting to happen or do they just regroup and get back into the groove (so to speak).
I look at a rider who looks to have all the time in the world, but a check of the times reveals blindingly quick laptimes.
I look for a rider who may not (and IMO likely is not) on the best machine, but instead is outpowered but able to catch, lead, pass and beat those with far superior equipment and do it with what looks like ease.
I look for a rider who only infrequently looks behind and then at a point of the circuit where it does not cost time (ie. where the circuit just travelled is in easy view).
It is hard to describe all aspects as above all else many are that pure gut feel you get from watching them live at a circuit as there you can hear their throttle application and gearbox usage, everything is 'nearer' when at the track.
But, just because I may see all this does not mean that they are what I term as 'truly exceptional talent' as they have to produce the goods time and time again, at different tracks under different conditions. If they are able to produce the same 'gut feel' in me after many months at different track in different races against different opposition and preferably on different bikes, then I start to wonder what we have. But ultimately, it is when that rider goes to a 'foreign' land that the judgement really starts to show as right or wrong.
Garry