I never attempted to read the "cycle" from the start nor envisioned reading them all eventually until recently (in the order/classification established by Balzac for the "complete" multi-volume edition, which is not the original publication order). I've read maybe 6-10 of his novels long ago, and in school we had read only some of his novellas.

Which ones are "the best" depends on tastes, though you can't go wrong with the famous ones (eg: Le Père Goriot, le Colonel Chabert). But it's a terribly varied body of works (several works are even non-fiction), from urban to agrarian drama to character/mores studies, philosophical novels to even roman fantastique (proto-Maupassant à la Le Horlà, you might say) to historical drama in the vein of Hugo, or Dumas (with more psychological/social depth). It also goes from very short novellas to long novels. Some of it recalls more Dumas or Hugo, some of it Zola, some of it Sand and co. (and apparently Trollope... and elements/narrative/storytelling quirksof Balzac still find their way into modern novels).

For a taste you could well start with the beginning (which would be Scènes de la vie privée, starting with La Maison du chat-qui-pelote. It's all novellas at first and they're all in the public domain (you can also find editions of the complete thing with notes for like 99 cents as one ebook), some more interesting than others, but in them you find all the great themes developed in the later novels.

It pretty much established the style of Balzacian narrative that became widely used by others, with the narrator being extremely present and telling a lot (Balzac like Zola would massively irritate the "show, don't tell" modern school of fiction writing).

Again, for a taste the "big titles" are the safe bet with Zola, ie: Germinal, L'Assommoir, La faute de l'Abbé Mouret, eventually Nana - all of them are strong). There are weaker ones, and the story/characters/milieus of some novels are less interesting (this will vary a lot from one reader to another). Les Rougon-Macquart cycle is nowhere as long as La Comédie Humaine though, so you could easily start with the first novel (La Fortune des Rougons) that establishes the first generation of the family and the origin of its wealth and overall story (and on the political front, it's the coming of the Second Empire which is the socio-political background for all the other novels). The second novel has the story of two of the Rougon sons in Paris, though the main characters are actually far more the young second wife and the son of one of them (both are not in the first one), while the husband is more like the most important secondary character. It's all interlinked if occasionally loosely so, but Zola wrote each novel so they can be read perfectly well as stand alones, the family continuity and other various references to previous events just become background details for the readers then, that's all (in other words, they enrich the reading experience if you've read the previous novels, but they're not necessary to comprehension nor are they presented to get anyone lost).

Like Balzac I had read several Zola novels before, but it's the first time I read the whole following the intended order. It's more interesting this way, I find (or maybe I didn't enjoy Zola as much when I was younger), though it's not quite "a series", more like an ensemble of very different works, with recurring characters. Like for the Trollope cycles, it's directly inspired by Balzac's idea of a framework and recurring characters playing central or totally background roles depending on the novel, but it's got a fairly different prose and vibe (the only thing about Zola that's a bit laughable now are his outdated notions about heredity and genetics, but though to him this was a central element of the cycle, it's easy to make abstraction of this aspect).

I mostly wasn't in the mood that day for a series of chapters on anglican Church stuff, but I'll certainly pick it up where I left before too long.