To me, most of the dragons are nearly worthless, on their own. It's much better to combine them and get multiple buffs, and even then, it takes at least two.
To me, Water and Air are a great combination as they grant safety, and Fire and Earth combine well to take towers and future objectives.
However, getting just a single buff of one type feels a bit lack luster. If you ever get three of the same one though: HOLY SHIT IS THE POWER REAL
Three Water dragons is absurd when you decide you need to keep fighting, and the three Earth dragons will turn any team into a siege comp. Three Fire is a nearly free Rabadons passive, and three Air makes a mobilty boot enchantment, on top of your actual boots.
So to me, the dragons are nearly worthless on their own, and no matter what, you should always prefer that you get the buffs instead of the enemy team.