|Only the strong will survive.|
For me, these armies are by far the most powerful compared to everyone else. Other armies have gimmicks right now, and that's pretty much it. You have flying circus Daemons, deathstar Eldar + DE allies, Nob Bikerz, and other stuff out there, but these armies stand out above the rest. Some armies saw more boosts, some saw some nerfs, but in the end, everyone made it out of 6th Ed. alright. On a whole, every army improved in some ways, and lost in some ways and it looks like everything is going to be balanced at the end. Except..
...These three armies that I listed above. I'll start with Necrons because they're the talk of the town right now. Being the last book before 5th Ed., it's a no-brainer that the army was designed with 6th Ed. in mind. In fact, it's such a no-brainer that this was their design philosophy that when the FAQs came out, Necrons received virtually no nerfs of any kind. They were already a pretty strong and well-balanced army at the end of 5th (even taking home some serious tournaments), but they were nowhere near as strong as they are now. A lot of their units saw incredible buffs in the way of Flyers, their Deathmark sniping is just insane with the new rules, their HQs/Wraiths and Destroyers saw epic boosts to their effectiveness, they can turn on the lights vs. Night Fighting on their turn, and their Gauss drops vehicles much better than they did in 3rd Ed. In short, Necrons lost nothing, but gained everything they need to be a dominant force in the meta. The entire book is strong, with virtually no bad choices and plenty of competitive options. In a way, they remind me of Space Wolves in terms of sheer points to competitive options, except they're not as cookie-cutter as Space Wolves. Instead of seeing RBs and Long Fangs, you will see a lot more WTF options all packed in one cost-effective list. This is what sets them apart on top of their other little goodies.
However, I don't think Necrons are the strongest. I think they are the army that benefited the most from the new rules and lost nothing, so that in combination with an already good book is what makes them powerful.
Next, I'll talk about a book that didn't receive as many buffs as the 'crons, but made it out just fine in the new edition. That book is none other than Grey Knights. The entire army turned from Daemonhunters to MEQhunters when the re-write came and that pissed me off the no end. Everyone had Force Weapons, the Rifleman w/ Psybolts is just out of control and there's so many powerful psychic powers and unstoppable [grenade] effects that made assaulting the army impossible. They are costed better, shoot better, assault better and just preform better than regular MEQ (especially in MEQ matchups), making GKs the best marine book out there. Now that the entire army has access to the coveted 2+ armor save and sport one of the strongest psychic lores out there i.e. Divination, the army is still remarkably strong. Their access to Stormravens also helps the flyer coverage of the army and it seems like there's nothing GKs can't do better than their MEQ counterparts.
With GKs being the best marine army, it's safe to say there's only one spot left among the most powerful armies of 6th Ed. Ever since the book came out, it's viewed as one of the most controversially underpriced, maladjusted and powerful army books ever written in 40K history. That book is none other than Imperial Guard. With the coming of Hull Points in 6th Ed., a lot of people claim that that would be the death of armored IG units. Oh man were they wrong. Mech IG is the damn same; gaining greater mobility, the ability to Overwatch and bonuses to Rapid Fire Plasmas. It almost seems like IG are there to break the rules of the game entirely, making 3rd Ed. Eldar seem tame in comparison. There's orders for accuracy, denying cover and making enemy units drop dead in their socks. They have access to the most underpriced piece of shit excuse of a unit I have ever seen in the form of the Vendetta, and now IG can take allies to compensate for their various weaknesses. All across the board, large blast templates now fall with full effectiveness, reserves are denied or provided via Officers and enemy fliers can eat shit from the Hydra flak, in addition to the now purchasable Aegis Defense Line. They shoot as good as marines, score better with more bodies, pour out 3x-4x the amount of heavy fire and seemingly have answers to everything you put on the table. I read this somewhere on the internet a long time ago about IG, and I still think even after these long years the same feeling applies: "No other army in the game makes your army feel as inadequate."
On that note, I'll close off this post. To me, it's pretty clear which armies are just outright better in this new world of 40K. The NOVA Open is coming up and its our first 40K major where players all over the world can try their cheese. I can't wait to see the results as I predict my analysis here will be well-justified in the final standings of that tournament. Of course, there's always the concept of bad draws and gimmicky lists coming up on top. Only the future can tell if that's the case.