Hi, I'm a guy who watches/used to watch Winx. I first watched it when I was six to eight years old, back in 2005-ish. I had three sisters so after school, they'd take command of the television remote to watch their shows. That's how I got introduced to Winx. I was a kid, so I didn't understand most of what was going on, but I loved the action and the magic.
We only saw the first three seasons - Rai English - before we moved elsewhere (pretty fortuitous as it turns out). Now, over a decade later, I have just marathoned through the first three seasons in two weeks, for childhood nostalgia more than anything else.

So, my opinion:

1- First season's great: nice soundtrack, magic, epicness, fighting etc.
2- Second season's okay. Loved Layla, didn't like Darkar, pixies were okay. Loved the new Red fountain.
3- Third season: best season. Loved the sacrifices (especially Flora's), Valtor as a villain, the Valtor-Trix relationship, the humor.
4- Got up to the tenth episode of season 4 before I quit in frustration. Hated the setting of Gardenia, hated their shop, their nauseatingly cutesy pets, the pathetic villains, the focus on girly stuff rather than adventure. Bloom's Mary-Sue'ness was unbearable. Hated the new soundtrack.
(And from what I'm hearing of the other seasons, I don't think I'll be watching them either. Makeovers, new voice cast, increasing focus on clothes and relationships? Instant turn-off.)

-Flora's my favorite fairy. A girl as kind and sweet (and pretty) as her is a rare gem. Flora's inspired me the most.

-Layla and Techna tie for second place. Layla for her sportiness, fighting skill, powers and wit. Techna because who doesn't love a hot, geeky girl?

-Of the boys, I liked Riven, Nabu and Helia. Riven especially in season 1 because of his complexity of character. Nabu 'cuz he's a badass wizard. Helia because I can relate to his quiet guy persona. If I could be a character in Winx, I'd be Helia, partly because he has Flora as a gf.

-Darcy's my favorite Trix. She's clever, witty at times, wears glasses, more calculating and calmer than her sisters and has the best powers. Griffin's my favorite witch.

-I didn't particularly care about the girls' clothes, but I will say Stella's Winx outfit was the sexiest of them all. (In fact, I think Stella might've been my first animated crush.) Techna's was pretty cool-looking too, especially as she had a paraglider? for wings.