Uncommon monocolor cycle: 6 mana for 3/3 makes them unplayable. Megamorph for 7 makes them unplayable. Giving +1/+1 counters is nice, but can only happen after you pay the whopping megamorph cost, and even then, only the green one can get help with additional counters if Hardened Scales is on the board. Rejoice, kitchen tables of the world.
Rare monocolor cycle: The red one (Thunderbreak Regent) has good protection and is only a 4-drop. The black one (Deathbringer Regent) is heavy as a 7-drop but it does have a Wrath effect.
Rare allied color pairs cycle: Harbinger of the Hunt could be interesting with a lot of mana laying around to pick off opponent's critters.
Mythic Rare allied color pairs cycle: Dragonlord Ojutai at 5 has an interesting card drawing ability. Dragonloard Kholagan at 6 can deal 10 life in a Grixis shell with tons of milling.
Uncommon allied color pairs cycle: 6-drop 4/4s intended as limited bombs.
Summary: As I expected, this set is full of constructed unplayable junk rares because Dragons by design are usually junk rares with a few exceptions. I can't see any of these Dragons making it in Modern either, but if I am wrong, no more than one can make it, and that would most likely be one of Dragonlord Ojutai or Dragonloard Kholagan. I am betting on Ojutai at 5 IF the remote possibility of Modern-playability does pan out.