Why ocean drake is actually the worst drake
We're going to consider the strengths of the dragon buffs when you have 3 of them.
Let's start with infernal drake because everyone loves infernal drake. Having a 24% increase to your AD and AP is just insane. It's almost like having a deathcap. No one needs an explanation for why this buff is strong; you just deal a lot more damage.
Earth dragon is also really strong but when it comes to taking objectives. It gives you 30% bonus damage vs monsters and towers if you have 3 and the opposing team has to keep a close watch over their towers, dragon and baron because your team will be able to take them in an instant. And if you ace the enemy you can threaten to end the game right then and there. By simply having more stacks of this buff than the other team your jungler can out smite the other jungler assuming they're same level.
Now for cloud drake, the buff a lot of people believe is the weakest, and ocean drake, the buff I believe is the weakest.
Ocean drake gives you a lot of healing; 12% missing health per 5 secs. This is only if you stay out of combat for 8 seconds. Offensively, this buff doesn't do much for you. It doesn't really help you win a game it just makes it so that it's harder for you to be poked down. If you take an empowered from Jayce's cannon you can heal it up through the buff but only if you stay away from a fight for 8 seconds. The opposing can take advantage of this.
Cloud drake gives you 45 bonus movement speed and allows to cross the map much quicker than the opposing team but this also if you're not in combat. This buff helps you offensively and helps you win games. It lets your team dictate when and where a fight starts. Your tanks/initiators can engage the enemy team much easier with this buff. Your carries can avoid being caught out with this buff much easier. Instead of healing up any damage they take, they can avoid damage altogether.
Cloud drake is a lot better than people believe because it helps you win games. Ocean drake is too passive to be better than cloud.