1) Bloomix: I think Bloomix is the strongest transformation because it conducts the magic of the Dragon Flame with the Winx's magic itself. In Season 6, it show how powerful Bloomix spells were.
2) Sirenix: Sirenix is a powerful transformation because it was a legendary transformation that could be earned only by the Sirenix Quest. You also had to cautious and quick to earn it because magic could be stripped away or you could suffer under the Sirenix Quest. It is based upon the powerful element of water.
3) Enchantix: Enchantix was supposed to be the "Final Fairy Transformation", so it kind of signaled that it was a strong transformation. Once you had earned Enchanitx, you are able to obtain Fairy Dust. Fairy Dust had the ability to break dark spells. The attack/offensive/defense spells also appeared to be able to cause damage or protection.
4) Believix: Believix is able to help people overcome their fears and strengths. This power was also able to defeat the Wizards of the Black Circle (with the help of Nebula and Roxy). Still Believix is a major transformation to help people's hearts.
5) Harmonix: Harmonix improved the ability of the Winx by helping them swim faster. It also was able to cause damage to the mutants and Tritannus when they used their spells to attack.
6) Magic Winx/Charmix: Magic Winx is a basic transformation. It is strong in its own ways, and the magic of this transformation allows the caster to use their strongest spell to appear (ex. Bloom and the Great Dragon).
7) Butterflix: It captures the essence of nature. With Nature's power, the Winx can use the force of nature to cause damage to the enemies. The only weak point is that they can't damage nature itself.
8) Sophix/Lovix: Sophix and Lovix are effective against the Guardian Fairies of Earth. If they are powerful against the major fairies, they can be label as strong. A weak point is that this transformation can only be helpful with nature. It doesn't entirely defeat an enemy.
9) Mythix: Mythix was able to defeat the monsters of the Legendarium and Acheron. A very weak point of Mythix is that it is rendered useless towards the Winx since the Legendarium is closed up for good for now.
10) Tynix seems to be the weakest transformation because all of the spells are conducted in the mini-world. It may slow down time when the Winx go, but it had no use in the regular world. Due to the fact that the bracelets were given by the Fairy Animals, Tynix has no purpose once the Fairy Animals won't be with the Winx anymore.
According to me
Mythix is the weakest as it only works in the lengdarium world and that to not very powerful.
Believix cause they first started firing spells in believix.
Harmonix cause it works both underwater and outside.
Sirenix because they behave like warrior fairies.
Bloomix cause its the power of the dragon flame.
Enchantix cause they've got awesome fairy dust.
For me its :
Magic winx beginner level fairy original form
Mythix only for legendarium.
Bloomix they aquired it so easily in the just the beginning episode 3
Butterflix not cool
Harmonix fighting skills improved
Sirenix they have to complete a quest to become warriors
Enchantix it was to be the final transformation.
Beleivix it got three sets of wings and powerful spells they used for the first time.