It's obviously Sirenix. The at least look like real fairies and Believix looks like that they are going for some fancy dress competition or halloween party. Believix doesn't works under water. The transformation song sucks for me, it's way too irritating for me to stand. Sirenix is stronger than Believix and Bloomix too as it was harder to earn, and what's difficult to earn is obviously better. Believix reminds me of those super obsessive and obnoxious pixies (except tune 'cause i like her) and Sirenix opened a doorway to the infinite ocean and Believix didn't gave them anything special except some pairs of wings which the winx barely use.
So all these things makes Sirenix my favorite transformation, and by the way, Believix was always at the bottom of my list.

Never really liked Sirenix.

My favourite transformation.

Not really a huge Believix fan, but it's okay.

I like this because all of them are equally unique while sirenix is almost the same Desighn though I love both

I'm not a fan of sirenix,they all look same outfits whereas believix looks cool..different outfits matching for each winx girls