Most of the Johto dex. I don't even think Ledian can be salvaged with a mega.
Also Unova has a lot of mediocre to shitmons that could use them, like Druddigon, Heatmor, and Sawsbuck off the top of my head
As for megas that shouldn't exist, most of them, especially the ones introduced in XY.
Half of them went to mons that didn't need them, like Gengar, Lucario, Charizard getting two, and the other half like Medicham, Manectric, and Mawile could have been actual evolutions with some tweaking to make them not broken as shit.
