Mine is Zotero. And I am using Papership app on my ios devices. Papership integrates very well into Zotero and lets you access, read and annotate your citation and files on Zotero. If you use Papership also on your mac, which has recently become available, annotated files in your library synch are seamlessly updated on all of your devices.
Since I have good reader app on my iPad, which also lets me use my ios device as a regular usb storage, I did not buy the annotation package ($4.99) of Papership app. I also observed a glitch during free trial mode. Moreover, Papership on mac is $9.99 for now.
There are many blogs and websites on the web telling how their life changed after they met Zotero or some other citation manager. That was the case for me as well. I just do not want to repeat here what you can find in other places. Also, there are many comparisons and how-tos on citation managers which are mostly made available by university libraries that might be useful for you as well: