Bloom.
It's like all she has on her side. Really the only reason she wins half the time is sheer luck...some sort of deus ex machina. Like how she earned enchantix without any sacrifice. Or how after the dragon fire got stolen in season one she somehow got it back (I don't even think there was a reason for it--it just came back). Or in season 2 when the Trix were winning and suddenly a wild Avalon appears.
Bloom definitely has the most luck on her side and its annoying af.
I think Bloom is the luckiest because she has the Dragon Fire and it was inherited not earned. And she earned her Enchantix but didn't save anyone from Domino, she just pretty much got mad at the Trix and her Enchantix came. And because all the antagonists are after Bloom at some point and they get what they want then the Winx or Specialists come and save her and have to do all the work.