For what it's worth I've played 70+ games on the no megas ladder with my current team and peaked at #2 with it. I've also watched countless ladder replays and seen what other good players are using, and I have a general sense of how the meta has shaped these past few weeks. These are some of my opinions from what I've experienced:
Landorus should be at B+ at best. Sorry guys, but it is so overprepared for in a tier already packed with Pokemon that can tank a hit and OHKO it (Azumarill and Conkeldurr come to mind) that it rarely accomplishes anything productive against good players. Anyone who seriously thinks Landorus is dominant against any kind of decent team setup or play style clearly is not experienced with what kind of strategies higher ranked players use. It just isn't that scary to deal with.
Further explanation- the lack of megas threatening to sweep in a moments notice really looses up restrictions in defensive cores, making using bulky attackers that take it on 1v1 and win much more viable. The extra slot granted by the loss of megas means it is also much easier to throw in something like Mamoswine or Weavile (already more popular thanks to how deadly Knock Off is) which can easily revenge kill it. Plus with how crazy good Sp Def Gliscor and Cresselia are at dealing with most fighting type/knock off spam, its not a surprise to see them also common as excellent counters to most Landorus sets.
Hydreigon should be A-. This Pokemon does so much work against opposing stall teams it should be mandatory on anyone banking on dragon spam to win matches. I am biased somewhat because I use it quite a bit but the ability to blow up 2-3 physical or special walls while doing a tremendous amount of damage to Clefable with Flash Cannon has always been a big help. Its also a great answer to Mew/Sableye/Gothitelle spam, something many balance or stall teams rely on.
Cresselia should be A+. It effortlessly shuts down a number of A and S ranked threats, and is one of the best answers to non-stab Knock Off despite being weak to the move. Being able to easily take on Landorus(both forms), Azumarill, Talonflame, Garchomp, Latios, Keldeo, Mamoswine, Slowbro, and plenty of others makes it an easy win condition once the few Pokemon an opponent has that can actually threaten it get worn down. Being able to throw around Thunder Wave also easily sets up the opponent for a late game sweep from something like Kyurem B and can save teams from dangerous Dragon Dance sweepers.
Conkeldurr should be S tier. Holy smokes, has this thing improved since standard OU. I am convinced that it is one of the most metagame defining Pokemon along with Azumarill and Gliscor, and this is primarilly due to Knock Off being much more deadly with Megas gone and considering how many awful matchups it had with said Megas. It not only has the bulk to tank a hit from the likes of Kyurem-B or Excadrill, it can continually regenerate all that health back thanks to Drain Punch. It has so many good matchups that its an excellent pivot on many teams and the ability to absorb status and revenge kill stuff like Bisharp or Weavile gives it plenty of defensive utility. It has all the strengths of Gen 5 Conk with Assault Vest giving it an extra layer of bulk that pushes it over the edge. Really great mon in general.
Klefki is B imo. Spikes are... okay, I guess? It was always a niche option even in standard OU and from what I've seen in No Megas nothing has changed. Yeah, Prankster T-wave is nice, and that typing is good defensively, but if you need a Dragon counter something like Azumarill is vastly superior. If Klefki had reliable recovery and wasn't Ground-weak (a death sentence in this ground heavy metagame) I'd rate it as a much better option.