I don't like the second season because of charlotte
1. She`s evil
2. She becomes a mermaid on purpose
3. I'm pretty sure her grandmother would have wanted her to be a good mermaid not a bad one.

Maybe season 2.

Season 1 is the best, in my opinion, and season 3 is the worst. Season 1 balanced the girls' discovering their powers with developing their friendship and their potential romantic entanglements and rivalries with others. My favourite stories in supernatural/superhero shows are always the ones at the beginning where the characters are really learning about their powers and trying to effect a balance with their new normal. Season 1 did a great job, and it also did a really good job in characterising Rikki, Emma, and Cleo.

Season 2 was bogged down by the narrative spending way too much time on Charlotte. I thought she was the strongest antagonist, but there was no need to focus on her at the expense of Emma and Rikki's screentime and development, for example. Too much time was spent as well on Charlotte and Lewis' relationship, but the one good thing was that that provided a lot of introspection for both Lewis and Cleo, as they worked their way back to each other.

Season 3 was not bad per se, but it was not particularly good either, in my opinion. Without Emma, the show struggled with writing the trio without a leader. In a way, Emma's absence gave the writers a good opportunity to recreate a new dynamic between three girls, none of whom are particularly inclined to take up a leadership position. But they didn't do that; instead, the writing of both Cleo and Rikki's characters suffered, in that Cleo got Emma's organizational skills, her efficiency, and her need to know what's going on to develop solutions to problems, and Rikki got Emma's controlling nature, her naturally cautious nature, and her dominating personality. These are traits that make for a strong female character, but their Emma's traits; they are not Cleo's traits, nor are they Rikki's. Cleo and Rikki have always been my favourites, and I dislike how they were sometimes unrecognizable in season 3. Also, none of the new characters were particularly great additions. Bella was for the most part a mild character; she was amiable and a little shy, except when onstage, when she always gained a newfound confidence. But her character was flat, in my opinion; she had very little personality outside of her crush on Will (all of her conflicts were Will-related), so it was hard to get invested. Will was, all in all, a dull character, to be completely honest. Sophie was a terrible addition; she had no purpose outside of causing trouble between Bella and Will, and Rikki and Zane, and ultimately stealing the crystals form Mako. Ryan was the most pointless character. He had potential upon his introduction, but eventually just became Sophie's lackey. Sam was nice, and it was nice to see a renewed focus on the Sertori family, but in my opinion, the Sertori family was also hyper focused on. We didn't see Rikki's dad at all, nor did we know anything about Bella's family, except that they immigrated from Ireland.

There were some great things about season 3, don't get me wrong. Rikki's bond with the water tentacle was one of my favourite arcs ever, because I saw the old Rikki return, and I liked how cracks began to form in the trio, and how Rikki naturally cut herself off as she tried to navigate her connection to the tentacle.
I thought the resolution of the water tentacle story-line was a bit rushed, but overall, the idea that water would "attack" the girls was a really interesting premise. Overall though, season 3 was a big disappointment for me.