Why neon has highest electron gain enthalpy? Why not other noble gases?
I believe it should be helium!?

Dear Student,
Neon is having highest electron gain enthalpy ​it may be because 

 the valence shell of helium is 1s,having the lowest energy therefore it will be easier to put one more electron in the next 2s subshell. 

In case of  Neon  we have to first overcome the 2p energy level and  it should have enough energy to loose when Ne accepts another electron in 3s that is  much higher energy level.

therefore Neon is having higher electron gain enthalpy.
Regards