The short answer is: probably Rebels
The slightly longer answer is: It really depends on what you’re looking for.
The full answer: Rebels and The Clone Wars are very different in scope and tone. Each is worth watching, but where to start depends on what you’re looking for.
Clone Wars takes place during the titular Clone Wars, between Episodes 2 & 3. It is much more an ensemble cast, with Obi Wan, Anakin, and Anakin’s padawan Ahsoka being the primary, but there is about a dozen or more what I would call major characters. During the course of the show you may go multiple episodes where Anakin, or Obi Wan don’t appear at all. The show does, however, use this to tell stories you couldn’t do with a strict main cast focus. Episodes that emphasize the relationships between clones (who have distinct names and personalities) drives many episodes, minor Jedi become major focal points, the universe is expanded and explored.
It also has the best depiction of the actual realities of war in the Star Wars 'verse. It makes the economic and political factors of the movies that, quite frankly, were dull and horribly handled, and makes them interesting and exciting on their own merits. The problems of war, such as starving populations, the political and social pressures, the cost in lives and money, all these are depicted well. It also brings out the role of the Jedi as generals, and haves it make sense. Plus the tactics of battle are done as well, or better, as many actual war movies. There is a lot of care and attention to how engagements occur, and you get the sense of how the different army doctrines operate.
Finally it does much to really show Anakin as a fully fleshed out character. His relationship with Padme is given a real sense of care, his descent into the dark side sold. You see Anakin as the great general and war hero. You see him as this exemplar of the jedi knights. You also see how his choices and relationships set the stage for his fall, even while he, and those around him, remain oblivious to the seeds of his destruction. It also gives Obi Wan a love interest, which provides wonderful contrast to Anakin. It shows the path not taken, and the costs and pain involved in that.
Basically it takes every potential, every promise, every thread and relationship that the prequels set up, and then squandered, and it fulfils that potential. It does this while paying homage to the old serials that inspired Star Wars in the first place, right down to the narrator. It is Star Wars as the best form of serial it could be.
Rebels is much more focused on a smaller narrative. It is a more intimate story, one that eschews the grand sweeps of galactic war for a much more personal story of a small band of outlaws. At the show open they are not part of the Rebellion, in fact the Rebellion does not formally exist. The crew of the Ghost each have their own stories, each their own personalities, and each their reasons for being there. Think Firefly.
As such the smaller focus enables much greater emotional payloads. This small scale enables the relationships to be explored in greater detail, and done so with an often deft and subtle hand. It may seem weird to describe a cartoon in the Star Wars universe as ‘subtle’, but often the histories of relationships between the crew are exactly that.
So by the time the Rebellion gets going, and by the time the crew joins, the groundwork has been laid. The evolution of the characters really comes through and visibly impacts the narrative. Choices and actions grow organically from who these people are. That way when old friends from The Clone Wars come up, they are also brought through to new and interesting dimensions.
Plus the story does a great job of expanding on the force mythos, and does so in the same ways that KotOR 2 does, by blurring the lines between light and dark. Rebels also has higher highs than the Clone Wars. When I said that the ending of season 2 is as good or better than Star Wars has ever been, I stand by that. Lets just say that the appearance of several sith lords from the movies has a HUGE emotional impact. It is worth watching the series to see this alone.
Rebels has higher highs, and does not have the type of occasional clunker that Clone Wars has. It is a more consistent series, and generally maintains a higher level of quality.
Now there is one reason to watch Clone Wars first, or at least concurrently. There are several characters that make important appearances in Rebels. Some of the impact of certain reveals, particularly in season 2, are diminished without at least some familiarity with Clone Wars. By no means do I think it required to watch the entire show, I watched season 1 of Clone Wars before starting Rebels, but at least a little. And whatever you do, do not start season 2 until you’ve watched some Clone Wars.