None. They need too much space. I've only 16 GB.
This is an iPad forum.
I've moved this over to the Off-Topic section (as it IS an iPad forum here .... and this question isn't a poll).
In addition, we have a sister forum that deals with the iPhone. It can be found here: iphoneforums.net.
Maybe you might want to ad that to your reading schedule, also? it's got great info on the iPhone and friendly members [too].