Here's my opinion on all of them.
Mega Sceptile
Very interesting. Unique typing, good ability and the ability to run both physical and special movesets (although that might change depending on its stats). I agree that it absolutely crushes Rotom-W, but I'd say it has more of a niche against rain, mostly for the same reasons it's so good vs rotom-w (4x resists water, immune to Electric).
Mega Swampert
First good physical Swift Swimmer. Super-fast EQ/Waterfall is really nice, a welcome option for rain teams, although probably still outclassed by Kingdra/Ludicolo.
Mega Slowbro
Probably gonna be pretty bad. Even with the stat boosts, a fairly useless ability and subpar defensive typing isn't looking so good for it. It'll still be decent at setting up Trick Room though.
Mega Sableye
Lower speed, increased defense, only 1 weakness and Magic Bounce? Sounds like one hell of a Trick Room setter. I think a set of Fake Out/WoW/Trick Room/Foul Play could be really strong.
Mega Salamence
Dear lord this thing is good. Draco Meteor/Heat Wave/Hyper Voice/Protect wrecks the whole tier.
Mega Altaria
Pretty much the same as Mega Gardevoir, probably higher Spe and lower SpA. Dragon/Fairy and Psychic/Fairy have similar coverage, so it'll probably come down to the rest of your team.
Mega Metagross
Pretty cool, has Clear Body to deal with intimidate switch-ins and the ever-formidable Tough Claws. Apparently it's getting boosts to every stat except HP, and it's getting a big boost in speed, so I can see this being a huge threat in the tier.
Mega Lopunny
With Scrappy it gets perfect coverage, it lacks Close Combat however. It also has access to the elemental punches which is pretty nice. A set of Return/Drain Punch/Ice Punch/Protect is its best bet IMO.
Mega Audino
Meh. Healer as an ability and decent defensive stats are all it has, maybe it'd be ok as a regular pokemon but as a Mega it's just disappointing.
Mega Diancie
Although it lacks Sableye's typing, it also has low speed, Magic Bounce and Trick Room to take advantage of. If it gets an attack boost, Diamond Storm could be pretty scary.O