Its not lionel, Clark doesnt give a hot if lionel dies. Its obviously one of Jonathan, Martha, Lana or Chloe.
Jonathan - Him dying would be a way to prevent him not being governor but since it was done in the movies, it may not be done here so as not to be predictable.
Martha - Pretty boring character as of late so could be up for death
Chloe - Her death could spur Lois to be a reporter and Clark to be more Supermannish
Lana - Since CLark tells lana his secret and proposes to her it pretty much ends the one of the story arcs of the whole series, so killing her could keep it fresh. Also its in the Fortress so closer to Jorel to do his business.