I don't have any thoughts on which Pokemon will have the most effect, but I will say I'm not that concerned about Ononokusu and Sazando considering that Salamence, Garchomp, Latias, and Latios can get fair shakes. Salamence is the slowest of those, and even with it alone there are important things it can do that the new Dragons can't. Some examples:
Dragon vs. Dragon: Salamence can coin-flip against itself and potentially end an enemy sweep if they have equal Speed boost levels (Dragon Dance vs. Choice Scarf, or neither one boosted), while the new Dragons wind up getting hit by Outrage or Draco Meteor every time in the same scenario. Even if Salamence doesn't have a Choice Scarf and has to come in on a boosted enemy Salamence, it can at least get in one final Intimidate--a small effect, sure, but still better than Ononokusu or Sazando accomplishing nothing. Even Flygon, though it isn't exactly awe-inspiring, at least has the opportunity to get in one good, solid hit against the new Dragons in town guaranteed if it gets in when they're not boosted or if it has a Choice Scarf.
Choice Scarf Iron Head Jirachi: A Speed+ Salamence with a Dragon Dance set up or only has to be scared of Choice Scarf Iron Head Jirachi outrunning it 50% of the time, but the new Dragons are always going to take the hit and risk the flinching.
The new 99-Speed Bug/Steel legendary: If this thing gains traction as a sweeper--and I sure don't see why it wouldn't--then Salamence can still Fire Blast it without worrying while the new Dragons will have problems. Besides being outrun in the first place, Sazando is weak against Bug and Ononokusu's low Special Defense won't like Download-boosted 120 Special Attack.
Ninetales: If Ninetales becomes as common as, say, Hippowdon due to the weather (better weather than Abomasnow, surely) and plays Choice Scarf hit-and-run with indirect moves like I think she will, then Dragon Dance or Choice Scarf Salamence gets to flip a coin on whether it needs to face down Will-O-Wisp or Toxic or Grudge while the new Dragons will definitely feel the brunt of those moves.
Now we actually have a Ground/Flying at 101 Speed giving extra incentive for Garchomp, and assorted 108-Speed legendaries who outrun those two but can't match Latias and Latios. I just think that unless the day comes that all four of the powerhouse fourth-gen Dragons are sent off to Uber for the second time, the new Dragons are going to have issues competing with the old ones due completely to Speed.