Cloud Drake = a bit of movespeed + tripled out of combat movement speed. (You lose 2/3 Cloud on dealing or taking damage)
Mountain Drake = damage to structures and monsters (a jungler's best friend, but only really helps laners if they reach the towers)
Infernal Drake = Damage stats (dps best friend.)
Ocean Drake = bonus regeneration when not hit by champions or towers in the last few seconds. (A support's or tank's best friend, and also, you don't lose this until you take damage.)
See...the thing with Ocean Drake is that it's essentially Warmog's, but trades some of the hp regen for mana regen. Would you say Warmog's is trash on tanks? It doesn't regen you right away, after all...but if you fight your fight and go ward out the enemy jungle, then by the time you're back in lane, you've regained most of the hp you lost. In the case of Ocean drake, you have your mana back too. Guess who never has to leave lane again?