Is it worth to visit all the towers: Oriental Pearl, Jinmao, World Financial Center, Shanghai Tower?

Actually, you don't have to go visit all the towers. You should choose one. The Oriental Pearl Tower is highly recommended. Apart from sightseeing, you can have a dinner at the rotating restaurant. It is quite delightful and unforgettable! As I know, there are souvenior shops nearby the towers.

Yes, the Huangpu River Cruise is worth riding. Standing on the cruise, you will see the traditional western style buidings on the western bank, and the skyscrapers on the eastern bank. :)

What about the Shanghai Tower? Is it also worth the view and time?

Yes, it is also worth to visit the Tower. Nowadays, it is the tallest building in China with more than 2,000 feet high. If you have enough time, you are advised to tour around.

Hey, if I visit SH on my way to Harbin, I will stay 6 hours on layover in SH. Do I have enough time to explore the Bund and Lujiazui where the top three towers are?

Also, if I have some time, do you recommend visiting Sheshan Mountain? Thanks.

Well, according to your short stopover, you are just advised to stay at the airport because you need to go back to the airport at least 2 hours in advance before the departure. Thus the time is not enough. There are several cafes and duty-free shops inside the airport. You can kill the time in these places. :)