A lot of those boats aren't going to be in the game for quite a while. And yeah, rarity doesn't usually mean much, except how hard they are to get.
Anyways, some of the good ones to raise would be all the starter boats for their eventual retrofits. Pretty much best in class. Laffey and Ayanami especially. But that is quite a ways off.
Enterprise is the best solo carrier with her skill that grants double damage and 100% evade. She will destroy the enemy with ease. Can be somewhat trolly in pvp due to that evade.
In terms of boss drops, Akagi and Kaga make for the most devastating carrier combo in the game.
(.75% drop rate for both though, have fun with that)
Since their skills boost their airpower when together. Also, they get to have their first launch ready 50% faster than everyone else. So that can ruin someone's day in pvp.
Illustrious is up there for sustain when doing higher level maps as she grants a shield to the escort fleet(the one you control directly) based on her hp whenever she launches her planes. Which can help a lot if you find yourself dying a lot.
Unicorn is your best light carrier support forever and ever. Also adorable. She heals your fleet whenever she launches her planes. She also grants a passive reload buff to your escort fleet. Reload affects how often your boats shoot.
Shouhou would be her IJN counterpart. These two are the only boats that can heal at the moment in the game. (Well, Vestal can too, but it is HoT)
Hood is like the strongest BC at this point. Since her barrage is absurd and if it procs, everyone reload faster.
Portland is one of the strongest Heavy Cruisers period by virtue of her statline and being a gunboat. (She can outclass SSRs like Takao and Atago yes...since they are torpedo boats) Even more so with her retrofit.
On a similar note, Cleveland is good for the same reason as a light cruiser.
Helena is great for granting like 40% extra damage for 10 secs due to her skill.
There's still more, but that should be enough for now. Also, San Diego is a meme.
Sidenote, your movement speed is based on how heavy your fleet is. So a destroyer only fleet would certainly move faster than a heavy cruiser fleet. The Beaver emblem is an aux equip that speeds up movement, but that is from completing the Little Beaver collection. (Hint hint raise them)
