Tricky, but I'll give it an honest shot. My opinion of them as of right now would be:
1. Salamence. The obvious powerhouse that we all know will wreck lives and destroy worlds.
2. Sabelye. The dark horse that people NEED to understand how amazing it is.
3. Slowbro. The other super-popular mega that really is THAT good.
4. Metagross. It's speed boost and new ability make it a frightening terror.
5. Lati@s. These two get enough of a boost to become that much more dangerous. Watch out for them.
6. Altaria. Dragon/Fairy is a great typing, and with pixilate, opens a lot of doors for Altaria.
7. Lopunny. This thing is gonna be good... later. I don't think we've even begun to tap it's potential yet.
8. Swampert. Rain. Rain. Rain. Rain. Rain. Swift Swim. Rain. Rain. Smash things. Rain. Rain.
9. Gallade. Good boosts will make him quite usable. Not the best, not the worst, but usable.
10. Sharpedo. He'll be tricky to pull off, but he's got some wicked power to play with.
11. Diancie. Kind of an odd transformation for her, but hey, it might work okay.
12. Sceptile. Can ruin rain's day, and has good attacking power, but still easy to KO. (4x ice weak? ouch.)
13. Steelix. Titanic physical wall with a mostly useless ability. Could work, but it'd TAKE a lot of work to pull off.
14. Pidgeot. SR weak, shallow movepool, lackluster stats even after mega evolving. Okay-ish is the best thing I can say about him.
15. Camerupt. Bad defensive typing somewhat offset by it's new ability still doesn't make him very good. Will be devastating in TR, but nowhere else.
16. Audino. Not a bad cleric, but why waste your mega slot on it? Seriously, I have two words for you: Sylveon. Clefable.
17. Beedrill. Yep. Dead last. As cool as he seems, we all know he's just way to terrible to ever seriously consider. 9/10 times he'll get killed right after mega evolving without ever getting an attack off.
Now this is just how I see it as of this very moment. We're still learning a lot about them and the games haven't officially launched, so this is subject to a lot of change and scrutiny in the days to come, so take it with a grain of salt.
EDIT: left out glalie somehow; It's late, call it a brain fart. Glalie's not that great, so put him tied at 14 w/pidgeot. Refridgerate-boosted ice type explosion is the strongest attack in the game now, but at the cost of your mega slot? Idk... doubt it's worth it.
I like the list however id probably rank them a little bit differently.
1. Salamence. I would agree with you that salamence is probably the most powerful of the new megas (which means ubers again for salamence) with all its increased stats, an almost base 200 flying return, reliable recovery, and an incredible movepool he is nearly unstoppable after one dragon dance. This thing is going to force teams to carry an ice type attack.
2. Slowbro: Incredible physical wall that is also can be an unstoppable calm minder. Although I stand by shell armor being a bit of a dissapointing ability, it is nice that mega bro doesn't have to worry about being destroyed by an unlucky crit.
3. Metagross: As Nappy would say "DEM PAWS!" I love this thing it has everything that metagross would need to get back into the OU metagame after the nerf to steel types.
4. Latios and Latias: Arguing about which one of these is better for me is like arguing if pepsi is better than coke. However, both these twins are going to be insane with their incredible special attack, bulk, and other aspects. If are already threats in OU right now they will be even more threatening in ORAS.
5. Sableye: Another annoying calm mind user who has plenty to distinguish himself from slowbro. With more special defense, magic bounce, wil-o-wisp, 3 immunities, and 1 weakness mega sableye can wreck teams that lack the appropriate counter to it.
6. Swampert: "I'm sweepin in the rain i'm sweepin in the rain" With the blessing of this mega evolution swampy know has a shot to make it back into OU. With the bulk, attack, typing, and ability, that dream world politoed may finally get some more use. And swampert too of course.
7. Gallade: Although I would have liked a better ability, gallade a mega evolution that is ready to du-u-u-u-u-ueeell! With a monstrous attack, swords dance, shadow sneak, and an incredible movepool you have got urself one heck of a sweeper. It can even run a decent bulk up set with the increased defense.
8. Lopuuny: This is one of the more overlooked megas in my opinion. Loppuny is almost like a physical mega manectric but with an better ability (I found intimidate to be a bit dissapointing). Now this thing can just wreck house with high jump kick and return without having to carry knock off like other fighting types. However, I do have one slight issue with Lopunny. Why can't it learn swords dance!?
9. Sharpedo: Many were a bit thrown off that sharp didn't get speed boost in its regular form. However, for me the biggest downside to mega sharpedo is that it got some pointless increase to its special attack which i felt could have been better in it's attack or even bulk. But still, strong jaw along with crunch and ice fang with a boost from speed boost can make this pokemon a pretty terrifying late game sweeper.
10. Diancie: I was a little dissapointed with diancies's normal form, but now i'm in luck because diancie's mega makes up for it. With it's incredible offfensive prowess and even decent bulk this pokemon is one I love to use whenever I get the chance.
11. Sceptile: Sceptile I also find to be overlooked. Although lightning rod is supbar it gives sceptile an immunity to thunder wave and the grass typing makes him immune to spore and stun spore. Scpetile is almost like a more offensive form of manectric that has a blistering 145 speed stat to outpace some of the top dragons in the tear and even its base 110 attacke can be used well with swords dance. However, a lot of weakness do hold it back especially with ice shard being a major issue.
12. Altaria: Altaria I though could have been better. I get that gamefreak wants to make some mixed attackers, but they just usuallly aren't my cup of tea. However, Altaria is save by it's typing and ability which can make it into a unique dragon dance sweeper. However, make sure you are aware of the other non mega dragon dancers such as dragonite and gyarados who can perform the role possibly as good as altaria.
13. Beedril: although beedril has an unfortunate case of wasting it's offensive prowess with protect, it still has a lot of things going for it. that adaptability base 150 attack is incredible and with a base 145 speed, it can still run an adamant nature to have a great speed stat. However, other flaws such as stealth rock weakness and priority being a problem, I can understand why you would
state that beedril is the worst mega. However, I believe the others aren't as good.
14: Camerupt: A bad typing is what severally holds this thing back a lot. However everything else about it I like. With it's increased bulk, attacking stats, pretty good abililty, and slow speed this will be quite possibly one of the best trick room pokemon. Of course, that's about it.
15: Glalie: Hey guy's, I found Michael Bay and Deidara's favorite pokemon! With that refrigerate explosion glalie has one of the single most powerful attacks in the game; its art is an explosion! Aside from that though, it does have a good niche as a spike setter; however without an item for a focus sash, it fails to do anything extraordinarily. Coupled with a bad defensive typing and not very impressive stats for a mega, it's gonna be one of those lower tier megas in the end. But I don't care because I want to make Michael bay see what true art is!
16. Steelix: Another dissapointing mega that does look cooll though. I am glad that steelix is now competitively viable, but doesn't get the ability or movepool to stand out as a defensive wall and it is outclassed by mega aggron in a lot of ways. However, u can play to steelix's more offensive side to be a good user of curse and gyro ball, but I would still pick another mega pokemon anyday.
17. Audino: While it's stats are good and it has a nice typing, as a fairy it faces stiff competition from the non mega fairy's such as clefable who also boast the same support movepool as audino and has two amazing ability's. Audino aready had a great ability in regenerator and game troll decided to give it an ability that doesn't even work in single's. Plus as a calm minder it faces stiff competition from slowbro and sableye.
18. pidgeot: Oh bird jesus, i'm so happy to see u finally get more powerful, but you were cursed by so many things. No guard is great with hurricane but it know leaves you open to stone edge, thunder, and blizzard all the time. a 121 speed is pretty good, but it's soo awkward because it would get outsped by greninja by 1 point! And you have been cursed by a terrible movepool for ur offense. No hyper voice and you could only use hurricane, heat wave, hidden power, and ominous wind? It's predictable and a waste off potential.
Overall though, I'm pretty happy with the mega's we got and I still hope there are more that we haven't discovered yet.