I'm about to go into Foretree Gym, so I'll comment on the Pokemon I've been using so far and their rankings (Treecko, Aron, Marill, Latias, Carvanha, Linoone, Tailow, Numel, Beautifly and Ralts). I've barely used the Exp. Share and haven't used anything too special that I've gotten with DexNav.
Treecko's been covered pretty well so far and I agree with its placement in A. I honestly think it could be S, but I understand the counterarguments against it, so I won't start that again. I waited until level 21 to evolve it so that I could get Giga Drain and it was well worth it and didn't take much effort or time at all. Its decent Attack stat allows it to use its great coverage in Rock Tomb, Low Sweep, Aerial Ace and Dig quite well, though it's going to be using Giga Drain most of the time, which is good since it's its best and one of its strongest moves throughout the game. It tends to get the right OHKOs and 2HKOs on most of its targets (maybe because mine had a Rash nature) and always hits first.
I second Aron's rise to B. While it has those nasty 4x weaknesses to Fighting and Ground and a Water weakness, its plethora of resistances make up for this and it has been very valuable to the team. While Mulan was wrong about being able to catch it before Brawly, it's not really inefficient to ride your Mach Bike down to the port by Slateport and head back to Granite Cave. Aron's gym matches are pretty good with Bulldoze for Wattson, Rock STAB for Flannery, a hilariously good matchup against Norman and great matchups against Winona and the twins. While I can see the impending water routes being bad for Lairon/Aggron, it excels pretty much everywhere else. It also has pretty poor speed, but it doesn't really need it when it's so bulky and it can be saved by Rock Polish or a Quick Claw. It's power and coverage are also good, with a great Attack stat and perfect coverage with Steel + Rock + Ground.
Marill is amazing with Huge Power and its wonderful Water/Fairy typing. It hits very hard with its physical STABs, has good bulk, is easy to get access to, and provides good HM support since it is capable of learning 5 out of the 7. Definite S.
Latias is really good and could be S or A rank. It is obviously easy to obtain since it's a mandatory gift Pokemon and comes with its Mega Stone right away. It can learn Surf right away as a coverage move and HM support and it comes close to the time where you get access to the Move Deleter as well. Dragon Breath and Thunder Wave are there for paralysis and are helpful for catching Pokemon and in battle as well. Recover helps to conserve Potions as well, making it a viable option. It may be because my Latias' Sp. Atk IV is between 0 and 3, but it sometimes misses some needed OHKOs with the lowish base power of its initial moves, though the Mega Evolution hits hard enough most of the time. Another minor point against it is that it's a pain to soft reset for, but I'm not sure if that should count. I'm kind of leaning towards A, but S is definitely fine.
I've barely used my Sharpedo so far since I just caught and evolved it, but it's been pretty good. The Surfing speed is godly when it evolves and it hits fairly hard from my short experience with it. Rough Skin is pretty good most of the time, though it can be a con sometimes when you want to catch a Pokemon (not that relevant of a con at all though). A is looking good.
Linoone is solid. It provides excellent team support with Pickup and HMs and can take out a Pokemon or two despite its weak firepower. However, it's still weak and may lose its luster as the game goes on. B is appropriate for it.
I dropped Swellow from my team around Wattson's gym as I didn't see much value in it after Brawly. It's pretty effective against most trainers thanks to its high Speed and good secondary STAB typing, but its gym matchups are pretty poor and, like Linoone, it doesn't hit very hard. It is one of the better users of Fly pre-Eon flute and one of the earlier Pokemon to get though so B could be okay, but I think a drop to C could happen as well. It looks like it will need Boomburst or Brave Bird to perform to its fullest.
Numel's good, pretty similar to Aron in some ways actually. It is one of the few Pokemon with a Fire typing in Hoenn and probably the best choice for a non-Torchic fire type thanks to its good secondary Ground typing offensively and defensively, moderate bulk and access to excellent moves early on in Earth Power and Lava Plume. Fire + Ground + Rock makes for perfect coverage, so that's also nice. However, its 4x Water weaknesses and low speed leave it useless on Water routes, so, it is less useful between Lilycove and Sootopolis aside from Tate & Liza and Shoal Cave (I haven't gotten to these parts yet, so I could be wrong about this). A rank is perfect for it though since it's the best fire type and is very strong.
Beautifly has surprised me in this game. While its Gym matchups are poor mid-game, but is very good against trainers and wild Pokemon. It hits surprisingly hard with Air Cutter and Silver Wind and Stun Spore makes it a great choice for catching Pokemon. Pokemon Rangers, Campers, Psychics, Battle Girls and Swimmers all fold to it and I haven't even seen its full potential since I haven't gotten Quiver Dance yet. I'd actually like to see it move up to A because it's just so solid in this game.
My experience with Ralts has been a bit odd. I caught a Male Lonely Ralts and intended on evolving it into Gallade. Well, considering Gallade's current E ranking, I think everyone knows how that worked out. :( But I'm stubborn, so I'm actually going to make myself suffer and wait until after Mossdeep to evolve my level 31 Kirlia into Gallade. Aside from that, Ralts/Kirlia is pretty decent because of its STAB combo, though their stats are lacking since they are frail and slow and they level up slowly (same goes for Aron, but it was less noticeable with her for some reason). I could see a drop to B for Gardevoir because of that early game stuff with Ralts and Kirlia, but I haven't used it in ORAS so I'm probably underestimating how great Gardevoir is once it evolves.
I'm gonna continue playing and refine my opinions for now, but I just wanted to get that out.