First of all - the riding is a lottery, the host is responsible for horses and the leader after two events draws the horses. In theory it means that host country's representatives have a handicap as they train on those horses, but in practic they do not always like the horse they get during the competition.
Laura Asadauskaite (Olympic Champion from London), Lena Schöneborn (Olympic Champion from Beijing), Zsófia Földházi (silver medallist of YOG 2010 and the biggest star of her generation), Qian Chen, Oktawia Nowacka are in my opinion main favourites for the gold medal in Rio. Another ones who will fight for medals are Samantha Murray, Donata Rimsaite, Margaux Isaksen, Yane Marques and Elodie Clouvel. Of course there might be some surprise, but they are the top now. Among men the best are Aleksander Lesun, Ádám Marosi, Róbert Kasza, Amro El Geziry, Pavlo Tymoshchenko, Riccardo De Luca, Valentin Prades and Valentin Belaud.
About the experts in certain sports - as I said riding is a lottery, most of the best run/shoot athletes are those mentioned above (except Marques and Clouvel), in swimming the leaders are Brits (Joseph Choong, James Cooke, Samantha Murray), Hungarians (Ádám Marosi, Zsófia Földházi, Sarolta Kovács), Egyptians (Amro and Omar El Geziry) and a few others (Woojin Hwang, Jiahao Han, Elodie Clouvel, Gulnaz Gubaydullina). Fencing is quite an tiring event as the ranking round lasts about three hours and the result often depends on the day.
I think that's enough for the beginning.