Say if I want to give my Pokemon [say Glaceon] an item that boosts Ice type moves, which would boost the power of an Ice type move [say Ice Beam] more, NeverMelt Ice or Icicle Plate?
It doesn't matter. They both boost ice moves by 20%.
Hope this helps!
They both would boost Ice Beam by 20%.
Before Gen IV, Type-enhancing items would only boost 10%, but since Plates debuted in Gen IV, this does not matter.