If the club wants "instant improvement" then Mourinho. His possible tenure could serve as a stepping stone for his eventual successor which should be a long-term successor. A bit like how Mourinho built the current fundament in RM that Ancelotti now has expanded upon and improved greatly. That would not have been possible without the fundament that Mourinho built.
Say whatever you want about him, what he did to Tito, his silly comments etc. but he is the man that we need if the goal is "instant success" or something close. He knows the club already as well and league. Has the experience, the achievements and can deal with the pressure better than anyone else.
Since that's not a realistic option I would go for Simeone but I doubt that he wants to manage another team in Spain. His next target is probably going to be the PL.
Heyneckes is too old and probably won't manage a top team again.
Klopp? I don't see him at the club at all. Not impressed by his tactical knowledge either.
Lopetegui? Depends on how he will do at Porto. So far he has not impressed.
Emery? No.
Garcíá? Could be a very interesting choice IMO. Impressed by his work in Lille and Roma.
Hiddink? No. Too old and he has not had any success with a top club for ages.
De Boer? Another interesting option. The question is can he take the next step? One thing is managing the by far best team in Holland for the past 3-4 seasons another thing is to manage one of the biggest clubs in the world.
Same story with Koeman. He has even a poorer CV than De Boer.
Benítez. Just no. He is a madridista and I can't stand him for some reason.
Cocu. Same story as Koeman.
Bielsa? A more exthentric Tata Martino that is yet to win anything of note. Not the right choice since we can't afford to make further experiments after Tata and Lucho.
Martínez is a interesting option. A tier below García though.
My top 5 rank out of those choices
if the primary goal is instant success.
1. Mourinho
2. Simeone
3. García
4. De Boer
5. Martínez
Ok, I just saw that the thread that I replied to was closed because this thread was made. My ranking is based on that other thread which can be seen here below;
http://www.barcaforum.com/showthread.php/12488-Which-manager-would-you-pick
When it comes to this current pool then I would obviously pick Pep in a heartbeat. If not him then Löw. Just to see what he can do at FCB and because I like him as a manager. There are rumors of Laporta contacting Löw. I doubt that he would succeed at FCB though. For some reason.
So that leaves me with Mourinho again.