The ones who always instantly jump out to me for being the most rhythmatic are: Evander Holyfield Ruben Olivares Santos Laciar There's a bunch I can think of who are almost rhythmless, but I don't think they count as the most rhythmic fighters ever, despite having great senses of rhythm.
Napoles is the epitome of a rhythm fighter IMO. How about Chavez? His movements were more subtle, but I think he qualifies.
Alfredo Escalera - El Salsero Joking aside, he did actually fight with an interesting kind of rhythm.