Earth definitely feels the strongest to me. You can see its effect and it makes taking dragons or baron noticeably easier. However, it is balanced by the fact that it gives no combat power whatsoever. The Earth buff feels good to have, but it doesn't feel OP.
It's difficult to tell how much power the Fire buff gives, but 8% AD and AP is fairly large. Simply _knowing_ I have this buff feels good. On live, the first dragon buff gives 6% instead, so this is a noticeable improvement. Getting more than one Fire dragon buff while the other team has none may actually end up being too overwhelming. Fire and Earth will probably be the most contested dragons in competitive play.
Water feels like it takes too long to work when you only have 1 or 2 of them, but when it activates, the effect is very easy to see. When it activates in the middle of a fight and saves you, it feels great. Worth noting is that tanks benefit a lot from having this since they typically take a long time to kill; this allows the Water buff to heal them while in combat. Changing the timer from 18/12/6 to 16/11/6 would be nice, but it might not be necessary.
Air easily feels the weakest. 15/30/45 movespeed outside of combat is simply not very strong. On live, the third dragon buff gives 5% MS. The minimum flat MS given is 16 (calculated for a champ with 325 base MS and no boots), but it **_persists while in combat_**. Having a higher base MS or buying boots increases the amount of MS given by the 5% value. Compared to the equivalent dragon buff on live, this is a big nerf. The values should be increased to 20/40/60 to make up for no longer persisting in combat.