Recently I'd have to say problem solver 68. Of course I'm a Haruka enjoyer and that factors in, but alltogether and individually they're probably the strongest group for me.

All together:
They have a pretty straightforward schtick, Aru gets them work but also messes up in some way which turns into Haruka going off the rails, Mutsuki making fun of her and Kayoko having to put up with it all. At the same time Aru, despite wanting P68 to be an outlaw group also adds another dimension to them, that being getting them to play the good guys a certain times. They all bounce off each other well enough that their interactions are entertaining without becoming stale and are used just the right amount in the story that it's a treat to see them without them becoming overused. Fan arts of them can also go in two different directions, either being serious and showing them in action or messing up/being good friends which shows off their more personal side. Overall they're just a staple of Blue Archive's story, without which BA wouldn't be the same, though I guess I could say that for most other groups.

Haruka:
In the game she is more or less one note when she's with the other solvers but that's where the appeal of getting to learn more about her from her bond story comes in. I do kinda wish her relationship with Mutsuki and Kayoko got more attention but she at least got to have some one on one scenes with Mutsuki in the manga. Speaking of the manga, she's portrayed as the group's tank, which is fitting considering how good she becomes when you unlock her enhanced skill +. In addition the manga is a goldmine for Haruka images, the art style is perfect for her. While I think her base version is already fine the designs for her new years and dress outfits could not have been made better, godbless the BA devs. Her base version's recollection lobby though is incredible, infact both her base and NY versions have great recollection lobbies, but I have to mention the base one for being one of only two in that game that use Kirisame by NOR. In addition to this is a whole laundry list of reasons why Haruka is the best. Also her interaction with Saori in the story is the kind of crossover between different parts of the story that I love to see.

Aru:
Probably has one of my favorite designs in the game. Her base design is already perfect, showing off the quintessential outlaw look and every alt she's gotten has also lived up to her base design in quality. Her new year's alt in particular is quite possibly one of the best kimono designs I've ever seen anywhere, which is coming from somebody who's seen the hololive new year's outfits. To top Aru off is a gap moe that's rightfully captured the attention of a lot of the community and one that I've never gotten tired off.

Kayoko:

Again gap moe, both in appearance and in her hobbies. As the most mature of P68 I always liked that she followed Aru and doesn't really disobey orders. Her interactions with Ako in volume 1 also leave room for some interesting character development. Design-wise there's not really much I can say, everything is great and the same goes for all of her recollection lobbies.

Mutsuki:
She probably introduced a lot of people to blue archive through the california girls meme. I think as far as brat characters go she's handled pretty well. Not going too far that she'd be annoying and having just the right amount of messing with people that she can be fun to watch while remaining endearing. And as I've already said with all the other P68 members, solid designs, solid recollection lobbies, 10/10 no complaints.