This one is a no brainer. Azula would kick some serious butt. While Bloom has her silly dragon fire, Azula has fire and lightning. She has two elements, not just one. And she's an expert at using both. Unlike Bloom, who was completely hopeless and confused as to how to user hers. Moreover Azula was a child prodigy, she mastered 14 forms of firebending near age 8. Bloom didn't even know she had powers at that age. That said, Azula has far more experience. She's been working with her powers far longer and has had the skills to use them far longer.

The dragon fire may be the most powerful thing in the world of Winx, but not necessarily in the avatar world. Not to mention Azula's fire is blue (scientifically blue fire is hotter than orange fire). And then there's the fact that Azula took down Avatar Aang, master of all four elements, most powerful in his series. Bloom may be the 'most powerful' in her series (and I say this loosely because I think she's the weakest), but Azula was shown to take down the most powerful. This is canon. And as mentioned prior Bloom may be most powerful in her show, but she sure as heck isn't in other shows.

So how did Azula take down Aang? She's smart, very smart and manipulative. Bloom isn't all that smart; she's the one who went into Cloud Tower alone at night and let a book trick her into thinking she was a witch...in other words she's easy to manipulate. And Azula is a master manipulator. So there's another advantage Azula has. And that's not including her very carefully crafted plans that usually don't fail her. And if they do she's usually got a back up that works. Bloom pretty much dives right in and gets lucky; sucks for her most people in Avatar have stated that "Azula was born lucky".

Oh, and she can breath flames out of her mouth as seen in the finale. Never seen Bloom do that. Bloom can try to fly away, but Azula's lightning can reach a pretty far distance. And let's not forget she's also accustomed to fighting fire vs fire. Bloom is used to fighting ice. Fire vs fire is far different than fire vs ice. So there are more experience points to Azula.

Oh and did I mention Azula can breath blue fire! Because I think I'd like to emphasis this again. Azula is the only character in her series that can use blue fire (of course there are characters in other shows that can too) but I know no one in Winx uses blue fire...much less Bloom. And again, blue fire burns hotter than regular fire.

*Also adds something about how Azula took over an entire kingdom at the age of 14 with the help of only 2 friends* Said kingdom held strong against an entire army for 100 years. That said Bloom can't even take on Icy alone. Granted Icy's hella powerful. But Bloom can't handle a one on one battle where as Azula managed to take down the Avatar and an entire kingdom almost single handedly.

Plus Bloom is a little goody two shoes. Azula is merciless and would strike Bloom down without a problem or a hesitation to make. She killed (for a while anyhow) the Avatar. Bloom is to soft to deliver the killing blow and Azula would totally act upon that weakness, use it as an opening and take her down.

I am 100% confident that even a half out of her mind--hell a completely out of her mind, season finale Azula can take Bloom down without an issue. Even crazy Azula whose planning skills are impaired and impulsive can outwit Bloom. In this case she could probably take Bloom down on raw power and just that. And naturally Azula in the middle of a breakdown is thrice as ruthless. Any way you toss the dice, Azula has this fight.