- Lord Frieza
- Pein
- Crocodile
- Cell
- Madara
- Aizen
- The Rest
For me, that causes some annoyance. You don't need to go through a 90+ year plan to obtain all the Jinchuuriki, set up two false successors and then become a Jinchuuriki yourself, to be entertaining. For all the backstory Madara has, cheap attempts to excuse his behavior IMO, I find that character absolutely boring to watch. Whereas Frieza on Namek is timeless.
I won't include much on Super, because it's abundantly clear that 75% of Super isn't really well written. Even Ultra Instinct was revealed to be a last minute addition that conveniently lined up with throw away lines earlier in the franchise. I will say that Super's rendition of Frieza is entertaining, but he comes off as more of an imitation than the original.
Best group? I'm not as well versed in manga as plenty of others, so I'll have to narrow it down to Shichibukai, Espada and Akatsuki. And, almost immediately I think Shichibukai is removed because their dynamic is very, very different to what you can expect from the other two.
Both groups have plenty of wasted potential and really only highlight 3/4 members. The others get fifteen minutes of fame, if that, and then are ignored for the duration of the series. I will say that I think the Akatsuki are more interesting and slightly better written, because they aren't slaves to Pein / Obito. Whereas the Arrancar are effectively cannon fodder with some who have very, very similar personalities.
Some of the more ignored members of Akatsuki, such as Sasori, have hidden depth to them that you can't really expect from most Espada. The backstory regarding how he'd wait for his parents, then developing his disdain for waiting and eventually making a poison that doesn't kill you swiftly but has you agonize for days and wait for death is fantastic. Especially since Kishi didn't feel the need to expressly state much of that. Then you have interesting gimmicks for Sasori, where he changed most of his body and tries to cast away human emotions, believing them to be a weakness (once more stemming from his parents demise), the particular enthusiasm he expresses when describing how he converts someone into a human puppet. To even the fact he admires art and wants to make himself art, but only art that is eternal and won't go away. Just a really well crafted character with a lot of tied strings that keeps things together.
Then you have Ulquiorra, who ponders over human emotions because he doesn't acknowledge his own — years (decades?) of isolation in Heuco Mundo after being turned on by his unique Hollow-tribe has made him jaded to any kind of existence. He doesn't see a purpose or a point in much, until the girl who he's in the process of mentally damaging manages to impress him with her fortitude and hope. There's just so much less to go off of, and Ulquiorra is one of the most (if not THE most) prominent Espada. Even going to Grimmjow, the Hollow we're the next most exposed to, has pretty much one gimmick that is extremely similar to Kenpachi already, but cast in a more desperate light than Zaraki's carefree battle lust.
The Espada have cool powers, great designs and I love a good portion of them for that and their limited characters. But the Akatsuki effectively match them in design and powers, and their best written members have significantly more to them. Sometimes to their detriment, like Obito or Zetsu and even Pein. But you have a lot of that good, middle ground with Kisame, Sasori, Kakuzu and Konan.
Griffiths is just too vile