There is pros and cons for each club, it depends on what he prioritizes really, and we don't quite know that, do we?
Sporting glory?
Might as well stay in Juve: As big a change for CL as PSG in the coming years, and right now bigger than in RM. A La Liga trophy with RM would give him more recognition, though. A move to the PL could be more of a consideration - if Pep builds an awesome ManCity-side, then surely a PL-title would be glorious, but his chances in CL would be lessened. Also if the PSG-project doesn't result in CL-succes, it would be impossible for him to leave it, and he would be stuck in a very bad league.
Fame?
Might as well stay in Juve: He is undisputed starter, and could be a legend in one of Europe's most historic clubs. In PSG he would have a harder time getting recognized for his talent (harder to compare to others with same level, when in a bad league), but surely get attention when he scores screamers or make fantastic assists. In RM he would be famous - a true galactico, in a club with a huge focus on marketing and commercialization (and maybe a Bale-type they would do much to get to work). If he seeks worldwide fame more than sporting glory, RM surely would be the best guarantee of that. With a PL-side could also hype to the best player in the world, if he performs and challenges for the title. In that respect he would get more fame in a PL-side than here in Barca, as he would never be seen as the best in this Barca team - he could be that in the worlds best (hype) league.
Money?
Surely PSG could offer him the most: They are the best paid team in the world! (People tend to forget that - not RM with Ronaldo or Barca with Messi). They would move mountains to get him, but that is also the biggest advantage that PSG have - in my eyes nothing else counts in their favour (maybe stable management, but that is not guaranteed even there - and nowhere else to be exact). RM can compete with their money - and the greater attention could give him more commercial deals, which maybe would make them even. Manchester City could also pay insane amounts.
I don't know if Bayern are interested at all? Sporting glory will be bigger there, if he could establish himself, but the two other factors nearly the same as Juve, I suppose.
I'm not sure if we are interested as well? It seems like he is too expensive for us right now. We don't need him yet - maybe in a year or two, if Iniesta ever declines, but the timing is not good right now (would smash our well-functioning and balanced midfield-depth). I would only take him if he would be ready to earn less than half of what other clubs offer - to show that he is committed, and also less than, lets say Busquets, to know his place in this team. If he isn't ready for that he wouldn't fare well in our team anyway, IMO.