Who cursed poor Zeref? I was thinking of Mard Geer, since he's said to be more powerful than Zeref.  

But was it ever mentioned in canon who cursed Zeref to be immortal?
In chapter 436, Zeref mentioned that he must have been cursed by Ankhseram, the God of life and death, for researching resurrection magic (R-System, Eclipse Gate, and later Summoning Magic). One of the side-effects of this curse was that he became immortal.



As mentioned by Denslat, this is indeed metaphorical. At this point it is not proven whether a deity exists in the Fairy Tail world. As far as we know, the curse was cast upon those magicians that had messed around with incomplete versions of Grand Magic (or Black Magic, depending on the translation). Zeref for using resurrection magic.


   Mavis for using (Fairy) Law. FT 449, FT Zero 11


So whether a real deity was upset or not, the people in the Fairy Tail world, call this curse the curse of Ankhseram, the God of life and death, regardless of its existence.