Hmm.. Well it really depends on what you want to use your ipad for as to what apps are best. I will say it is worth it to jailbreak. Gives you a lot more flexibility. I can't even say what jailbreak to use since it is always changing depending on what the latest iOS the ipad is running. I have an iphone as well, so these are some of the apps I use:

Reading: Kindle
Reading comics: comiczeal4
Instant messaging: IM+
for this message board: Tapatalk
Games: Words with Friends
Check lists: Todo
RSS reader: Reeder
reference: wikipanion
AVI player: CineXplayer