Bleach was never a series that had an abundance of face turns, unlike say, Dragon Ball, where half their heroes were villains at one point in time. In Bleach, there were some shades of grey involved, but most of the time villains were so devout in their beliefs that turning "good" wasn't possible.
There were a few that made the full turn and a few others that did due to circumstance. None of them ended up being buddies with any of the main characters, but they did play small roles in battles during the final arc vs the Quincy. This list will look into a few from each side.
10 Stayed Bad: Starrk
Starrk was the Primera Espada, yet one that is very rarely mentioned as fans' favorite. That doesn't take away from what a cool character he was though. His look was unique, as was his fighting style. Who doesn't love a guy bringing pistols to a sword fight?
The lack of care he had for anything was a great character quirk for a villain. Laziness is usually only a trait given to heroes, but it's clear Starrk would rather just sleep all day rather than fight. It made his fight with Shunsui so good, as they were very similar. The fact he realized he wasn't alone as he died, was the right end as well.
9 Turned Good: Grimmjow
Grimmjow is a hero by circumstance more than anything else. He still very much wants to fight Ichigo again, very Vegeta-like in that way. He is only helping the Shinigami because he couldn't have his rematch if Ywach were to win. It's simple reasoning that makes a lot of sense for the character.
Unfortunately, his turn doesn't lead to all that much as he runs into Askin Naak Le Vaar, one of Ywach's elite guard. In an effort to show off how strong Askin is, Grimmjow is more or less used to put him over, to use a wrestling term. It basically means make him look good.
8 Stayed Bad: Jugram Haschwalth
Haschwalth is the second in command under Yhwach in the Wandereich. He was one of the few that had both character and an actual arc. His back story with Bazz B gave him depth, as did the dialogue in his fight with Ishida, even if the battle itself was kind of boring.
He was the most loyal of the Sternritter, even becoming a stand-in leader for Yhwach while he slept, taking on his power. Even as Auswahlen enveloped him, meaning Yhwach was taking his power back, he stayed loyal to the end, stating that it meant he was still an asset. On his death bed he did take Ishida's wounds, but he still very much went out a bad guy.
7 Turned Good: Bazz B
Bazz B was another hero by circumstance. He joined with the Shinigami out of anger over Auswahlen being cast over the battlefield, returning many of the Sternritters powers to Yhwach. He made it his mission to confront and kill both Haschwalth and Yhwach for their betrayal.
Naturally, this didn't go too well for him as he never even made it to Yhwach. He confronted Haschwalth, much of his backstory coming into play here, leading to what you thought would be a pretty cool fight between the two. Instead, he was completely outclassed by the superior Sternritter, dying with a whimper rather than bang.
6 Stayed Bad: Nnoitra Gilga
Few villains in this series were as unlikable as Nnoitra was. Usually, Kubo tried to give them some sort of goal or trait you could sympathize with to a degree (at least before the Sternritters). Nnoitra had none of that. Instead, he was a chauvinistic pig the entire time he was on screen.
He held an immense grudge against Nel for the fact that a female was stronger than he was, a fact he could not cope with. His need to be the strongest was so important to him that an opponent sparing him was a direct affront to his pride. Their refusal to kill him means they don't see him as an equal. He's one of those characters you want to see die as grisly a death as possible.
5 Turned Good: Giselle Gewelle
Gisele was one of the creepiest Sternritters of the bunch. From her strange fascination with Bambietta, bordering on that of a stalker, to her ability to turn others into zombies via her blood. Add in the sadistic smile that was always on her face, even when chopped nearly in half, and she was never someone you wanted to fight.
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Unless of course, you were Yhwach, the leader of the Quincies. He was the sole reason she turned to the side of the good, wanting revenge on him for trying to end her life with Auswahlen. As expected, she didn't get that revenge, instead being taken care of in the span of a panel.
4 Stayed Bad: Yhwach
Much like Nnoitra, he was a villain you just wanted to see die. Not because of his personality traits, but because of how insanely boring he was as a character. Not only was he one of the least interesting characters in the Wandenreich, but he was also a horrible final villain.
It's crazy considering how amazing a Quincy vs Shinigami war should have been considering how much their history was built up. He just had no depth though, a fact that was made even more obvious when sharing screen time with Aizen, an infinitely superior baddie. He even ended up dying in a fairly dull way.
3 Turned Good: Gin Ichimaru
Gin was part of the trio who were the primary villains for much of the series, along with Aizen and Tousen. For most of that time, Gin was painted as a snake who bided his time before striking at his foe. As it turned out, that's exactly what he was, waiting for what he felt was the perfect moment to strike Aizen.
Unfortunately for him, he'd waited too long, Aizen's new evolved form being too much for him to handle. As he lay dying, we saw that everything he did was for Rangiku Masumoto, painting him as a tragic anti-hero of sorts.
2 Stayed Bad: Ulquiorra Cifer
Ulquiorra is one of the fan favorites in Bleach, mostly from the complexities of his character. As much as some fans would have loved for him to have a moment where he came back and fought side by side with Ichigo, it wasn't a fate meant to be.
It is for good reason, too, as Ulquiorra dying a villain fit the character far more than him pulling a Grimmjow would have. His sense of intrigue had been sated as he finally figured out what it meant to have a heart. There wasn't anywhere his character could have developed, especially with us seeing everything his release could do.
1 Turned Good: Kujo Ginjo
This spot could go to any of the fullbringers as they all had a minor, yet vital role in the Quincy war as heroes. Ginjo was the leader and thus the reason they (aside from Riruka) even shifted their alignment.
While the arc he was part of was incredibly forgettable, even feeling like a filler for parts of it, it ended up being important. If it weren't for the fullbringers, then Ichigo never would have been able to repair his Bankai after his first battle with Yhwach. It's just a shame they all ended up being mostly walking plot devices rather than actual characters.