Aronian. Winning the Sinquefield Cup was a major boost for him. The overall winner of the GCT (or highest in standings not already qualified) should also qualify for the Candidates, but clearly there are political issues at play, so that's unlikely to happen.
Candidates Wildcard

Nakamura is World #2 and came second in the Grand Prix series. He most certainly does deserve a shot.
It's exciting that Caruana, Nakamura and Giri are playing for the first time. Perhaps the organisers will select another Candidates virgin to balance this out?
Aronian. Winning the Sinquefield Cup was a major boost for him. The overall winner of the GCT (or highest in standings not already qualified) should also qualify for the Candidates, but clearly there are political issues at play, so that's unlikely to happen.
The Grand Chess Tour is more or less Kasparov's project and Kasparov has even declared that it's "better" than the FIDE Grand Prix, so I doubt Ilyumzhinov will use the GCT to pick the Candidates wildcard.
I would like to see Aronian or Kramnik. I don't think Nakamura deserves to be there.
Nakamura has qualified for the tournament, therefore he deserves to be there. End of story (and hopefully of your whining too).

Of course it also matters where the tournament will be held, for example if it will take place in Armenia, they will probably choose aronian. Ans so on.
The only thing that matters is which country that pays to get the wild card and organise the event. No one pays all that money to FIDE to buy a spot in the Candidates to then give it to a player that isn't their own, so my bet is that the Candidates as usual will be played in Russia, and that Kramnik will get the wild card.

Kramnik it is likely to be. Aronian is probably playing better, but has already been a candidate 4 times, and never really impressed in any tournament.
aronian\kramnik\grischuk\memadyarov
kramnik and grischuk are actually less likeley, because there are already two russian players.
I think Elajanov because he has won so many rating points.
Eljanov isn't eligible since he was below 2725 on the July list, same thing with for example Wei Yi.

Aronian can still qualify by rating if he wins and or Giri loses his games in London. And i don't think wild card would be Kramnik. He already denied invitation to super 3 grand tours.

Are there any rules prohibiting Carlsen to be elected as wildcard? He gets it, if he wins the Candidates there's no match! Better than a third Carlsen-Anand, isn't it?
We can now say with almost certainty that the Candidates field will be as follows:
Who then will be the Wildcard selected to round out this stellar field?
Top contenders are obviously: Aronian, Kramnik, So, Grischuk.
Less likely, but still possible: MVL, Jakovenko, Ding Liren.
Beyond these players it would truly be a wild pick to select anyone else.
Who then, would you place money on to get the pick, and why? Venue will possibly play a role, as well as potential movement in the ratings up until the announcement, which I assume will take place in December?