According to the OP, Immunity should be given to the pokemon you think is most likely to win the particular challenge, in which case I vote Dialga. The steel-typing gives it an edge over Palkia, who has literally no common sets that will be able to bypass the time lord. ScarfDialga wins against every Palkia set 90% of the time thanks to Draco Meteor, including opposing ScarfPalkia thanks to its natural bulk.
None of the other candidates have nearly as good a success rate.
EDIT: Shouldn't the pokemon granted immunity be protected from elimination during the NEXT challenge? If it gets voted for immunity, it obviously doesn't need protection from elimination from that same challenge.
In-Game Name: Anthony
FC: 3309 2165 3231
I use legal hacks.