I literally just posted this comment on a thread a couple rows up from this one. so i'll copy and paste it.
Riot provided numbers about 2 months ago showing with their own data gathered, infernal drag actually places about 3rd in most influential dragons in the game.
behind ocean and wind drake. it tied with mountain drake and elder took last because its spawns too late and gets taken too infrequently to give it proper comparison. meaning it fell middle of the pack and really wasnt even close to best drag in game.
Infernal looks good on paper, but look at it like this. Infernal drag boosts % AP and % AD. but if you are using a non auto attack based champion, then most of your AP and AD are only relevant in your skills.
lets say your Q has a 60% ratio, your W no scaling, your E a 60% ratio, and your ult a 60%+60% ratio. then that means You will only get 60% of the infernal drags buff on your Q and your E, no effect whatsoever on your W, and 120% of the dragons effect on the ultimate. That's it.
some champions have scalings as low as 20 and 40% in some areas while other champions may have their damage scale on things like health and mana pool ( ). making it so that blue boy is only getting half benefit from red drake as only half his AP scaling is tuned around him having actual AP.
The red drag is a trap in that everyone seems to THINK that the %BOOST it gives to stats is how much every champions damage goes up, but it isn't. since there are little to no kits in all of league of legends that 100% scaling anywhere outside of their ultimates. meaning other than auto attack based champs who can use the AD boost for better autos, everyone has to rely only scaling giving them a watered down buff that isn't nearly as effective as they thought it was.
So i repeat. infernal drag IS NOT the strongest dragon right now.