As a group, Off the Hook, but as individuals, the Squid Sisters. Though this defies the idea of S2 hero mode (Callie's metal health slowly deteriorating in her solo carrier causes her to eventually get hypnotized), I still like them better alone, if not alone then just reunited after a long time. Bomb Rush Blush is much better (in my opinion, please don't take offence) than City of Color, Ink Me Up, and Maritime Memory (note that Calamari Inkantation and Now or Never are both covers, not originals). In short, Callie is actually just a better artist, which I think Nintendo kinda messed up on. On the other hand, Pearl and Marina alone are... eh. Though I wouldn't say Into the Light and Color Pulse are great, together they can write things like Shark Bytes and Muck Warfare. @#$% Dudes be @#$% Sleepin' sounds weirdly like Fishing Frenzy, while Marina's only know solo work is a demo for Ebb and Flo, which isn't really their best song. I think that they need to be together to write their signature great music. Take the two songs I mentioned earlier. Muck Warfare is, in a way, a fusion of their two different styles of writing, and boy is it great. The way it starts with the duo's signature opening drum beat, followed by the synth always gets me excited when it plays during a Splatfest. The vocals are also an enjoyable hybrid of Pearl's upbeat style and Marina's more gentle style. As for Shark Bytes, it didn't fuse their styles, but instead played them singing different styles over each other. It starts with MArina singing something emotional but then starts to glitch out into Pearl rapping. Great transition. It is then followed by the singing and rapping playing at the same time with a few breaks for instrumentals and a bit more glitching. They wouldn't have been able to write that alone.
Yes, I just analysed their music for my opinion, and here's my conclusion: If I were judging it by fusion of individual talent, I would go with the Squid Sisters since my favorite song in the entire series is Tidal Rush and I feel like it shouldn't be. However, if you were judging by how they do together as a group, though there are a few outliers, I would undoubtedly go with Off the Hook.
But if you're asking who I like as characters...
Just Callie.
Sorry if this post is too long.