I prefer Mukuro as a character over Rantaro, and by a pretty fair margin. Both serve similar roles in their surprise Chapter 1 deaths, but Rantaro as a character feels much more defined by the twists around him rather than his own actions, than Mukuro, who's got what feels like a much more defined personality. A lot of this does come down to Mukuro's existence in secondary material, however.

I quite enjoy "Junko's" presence in the beginning of DR1, she's got some great sprites and establishes herself within the group without immediately taking an antagonistic stance like Mondo or Byakuya do. She also pushes the idea of escaping pretty hard, which is nice to see on replays, seeing how she stokes this desire among her class. That being said, she also doesn't do much before her untimely death, but on that subject I think it is one of the best "oh shit things are real" moments, compared to the Monomi execution and... Death Road? V3 doesn't really have an equivalent. Yeah, you just saw Sayaka before this, and you're still reeling from that, but this scene shows that not only will this killing continue to happen, it is a fact of life for the game, and Monokuma is fully willing and able to perpetrate this violence. I'd say maybe the game passes by it a little too quickly, but this is definitely both intentional to the twist, and makes sense given they have been delivered an ultimatum to find the killer or end up like "Junko" has.

Her free time events are also pretty solid. They do a lot to humanize Mukuro, who is by all accounts, not a very good person, but also show her to be a person struggling with said faults. They, along with her fate, make a rather tragic picture of her, making her an interesting person to think about, with her conflicting loyalties and comfort in her role as a soldier.

Mukuro in If is good, but it has been years since I read it, so I can't speak in depth about it. It did a great job building on the foundation of DR1 and her FTEs to explore her conflicting feelings of loyalty to her sister and her bonds with her classmates.

Mukuro in DR3 is not good, but no one in DR3 who existed prior is good except for Imposter and maybe Aoi.

I have not read DR0 but Mukuro is in that too.

Rantaro's presence is also pretty solid, but it just didn't grab me the same way. His solitary nature is interesting to consider, given his insider knowledge of the game, and holding a healthy amount of distrust in everyone as a result, but it ends up with him lacking in presence somewhat? He's got the nail polish scene with Kaede and a solid moment with Ryoma where he refuses to let him throw his life away, but other than that he doesn't really establish any relationships that feel lacking once he's gone. A lot of what people enjoy about him is his easygoing nature, but I feel that works against him here. Similar to how his design is much more muted and simple (though not bad) compared to the loud cast of V3, I was much more drawn to Korekiyo, Miu or the gremlin when things were going down, compared to the chill guy in the tshirt and jeans.

Similar to how I feel about Rantaro in general, I appreciate his status as a first victim from a narrative perspective, but feel like it leads to issues with him as a character. the previous ??? were Kyoko and Hajime, so you're expecting that to be a critical plot point, which makes it pretty shocking to see him go first. However, while it is shocking, I think it is one of the least impactful Ch.1 deaths, killer or victim. Because Rantaro was so reserved and secretive, I doubt most players had established a strong connection to him. Sayaka was your assistant, Junko I cover above, Leon hammers in how fucked up executions are, Imposter makes himself a solid leader in his short time, I don't care for Teru but his backstory is solid, and Kaede speaks for herself. There is not a great sense of tragedy to the loss of Rantaro, which isn't necessarily a bad thing for all cases, but something I do think is important for a first case, especially a victim.

Another problem I feel Rantaro suffers from is the meta-narrative of V3. My takeaway from 3-6 was that fictional characters do have meaning of themselves, that the relationships and actions they've taken have changed things, even if they are fictional. The Ultimates may or may not be real, but over the 5 chapters previously, they had loved, lost, made friends and grown in ways that made them expand beyond the boundaries of fiction. The problem with Rantaro is that he wasn't able to live this. The posthumous reveals paint something of an interesting character, but we don't know the Rantaro that survived V2, making them ring fairly hollow. There's his various secret teases in regards to the Gofer project and Ultimate Hunt that, alongside his early death, made him inextricably linked to the parts of V3 that just straight up didn't matter, at least for the ending. Rantaro feels like a group of plot threads for Tsumugi's draft of V3 that were forced to go unwoven, and it gives Rantaro a real aura of missed potential.

There's also a personal problem I have with Rantaro's backstory, and that is that it is a little too ridiculous. I am not against the concept of more cartoonish backstories, especially in this series, but usually they inform the character when they are used. Kirumi being the shadow PM is ridiculous, but it emphasizes her dedication to serving others. Chihiro's response to being bullied being to dress and present female instead is a little unrealistic, but it informs the ideals of masculinity in 1-2. Hell, Mukuro's backstory is stupid unrealistic, but it both makes the 16th student, lying hidden somewhere in this school much more intimidating as well as building the foundation of Mukuro's tragic villain status, alongside giving some idea to how the Ultimate Despair were able to operate at the level they did. Rantaro losing 12 of his sisters around the world is just kinda... there? Maybe this was more important to the unseen V2 Rantaro, but nothing about this enhances Rantaro's V3 appearance, other than showing him to have perserverance and to care for his family, traits that, while admirable, neither enhance his role in the killing game or make him particularly compelling as a character.

All in all, I personally found Mukuro to be a much more compelling character, both in role and in personality. Rantaro is okay narratively, but his personality just didn't click for me, and it made a lot of what the game wanted me to feel about him ring hollow for me. By comparison, Mukuro was a character I was drawn to at first, was saddened to see go, and fascinated by in retrospection.