Bear in mind that since Gen7, mega evolving comes with the highest priority. This changes the tiers of the mega evolutions somewhat.
I would like to remind everyone that right now we have the mega stones for the following pokemon:
Heracross, Pinsir, Banette, Abomasnow, Audino, Glalie, Camerupt, and finally Steelix (1/3 people get this)
The remaining are for the following pokemon:
Venusaur, CharizardY, CharizardX, Blastoise, Alakazam, Gengar, Kangaskhan, Gyarados, Aerodactyl, MewtwoY, MewtwoX, Beedrill, Pidgeot, Slowbro
Ampharos, Scizor, Houndoom, Tyranitar
Blaziken, Gardevoir, Mawile, Aggron, Medicham, Manectric, Absol, Sceptile, Swampert, Sharpedo, Altaria, Salamence, Metagross, Lati@s, Rayqay
Chomp, Lucario, NoPunny, Gallade, Diancie
So far we have received some of the worse (Audino, Glalie, Camerupt) and some decent ones (Pinsir, Heracross, Abomasnow)
I believe it's 99% certain we'll get Sharpedo, especially because we already got Camerupt.
Power-wise, I believe M-Ampharos and M-Pidgey are the next in line, being less useful than Pinsir/Heracross.
Also, it's quite possible we get one for Scizor since we're definitely not getting Scyther before we reach the Victory Road.