Futaba: RANK HER TO MAX AS SOON AS YOU CAN. She's a great navigator, honestly. She'll give you buffs in battle and restore hp and sp. She'll also swap out dead party members for healthy ones during battle. She has no skills outside of mementos/palaces.
Haru: Great for Heat Riser and heavy hitting with psy attacks. Pretty sure she's a decent hitter with phys but I can't remember if she has any on her. I swap her out for Makoto the second I get her. She's just great for boss battles. Love her. She grows vegetables that will heal sp/hp.
Akechi: Ranks up during story. Don't bother buying any gear for him. He's good with debilitate when you have him, so he's good for the twin battle, so I recommend doing the twins and getting to lvl 99 when you have him if you wanna do that. You lose him after Sae's palace so don't get any gear for him unless you plan on using him for the Sae boss battle or twins, or just her palace in general. You never get him back on the team. and getting to lvl 99 when you have him if you wanna do that. You lose him after that palace so don't get any gear for him unless you plan on using him for the the boss battle or the secret battle, or just her palace in general. You never get him back on the team.
Now onto outside of palace/mementos social links:
Sojiro: Great social link for the curry and coffee unlock. Sucks that you have to wait a bit to get the rest of his social link up, though.
Takemi: Great for the medicine and SP Adhesive. Recommend getting her social link up for cheaper prices and more medicine/items. Those are important.
Iwai: Great for the guns/armor/weapons. Also Recommended getting her social link up for cheaper prices and being able to customize guns.
Chihaya: I never had a use for her in any of my playthroughs. Her abilities are good for the readings, though. It's great for deepening bonds with your other confidants, so that's a plus. Just be prepared to loose 100,000 though.
Ohya: Her abilities are great, especially if you have trouble sneaking around palaces. As she ranks up, the power of her ability increases until each time overnight, your security level drops to 0. So, if you're still learning how to sneak around and not get caught at this point, she's a good one to rank up.
Shinya: Down shot is a great ability to have. It lets you fulfill a mementos request, and it's great for shadows you want down, but have no weakness to anything and Morgana just isn't getting it knocked down. It also increases the number of bullets you can carry, and you can set off a warning shot to scare shadows in an interrogation. Great confidant.