There are many different brands and flavours of shisha out there so recommending anything in particular is rather redundant as everyone has different tastes and their own personal preferences. It really depends on what you're looking for. If you have a sweet tooth and like sweet fruity flavours then I would suggest brands like Al Fakher, Starbuzz or Social Smoke (there are many other so I'm just naming a few). Nakhla is just as good but it is an unwashed tobacco meaning it has a slightly higher Nicotine content.
There are also traditional flavour shisha's available for people who don't enjoy the sweet fruity stuff but prefer the more spicy and complex tastes of traditional shisha's
I would suggest checking out the tobacco/shisha review section of the forum so that you can get an
idea about what flavours are worth checking out: http://www.hookahpro.com/forum/forumdisplay.php?f=20
With regards to specifics about nicotine levels, types of tobacco leaves used etc. all I can say is google is your friend.