Is it just based on their back catalogue or judging the company as a whole? If the former, probably "Penguin". If you're wondering why I put that in quotes, here's why:

• Ace Books • Alpha Books • Avery Publishing • Awa Press • Berkley Books • Current • Dial Books for Young Readers • Dutton • Dutton Children's • Firebird Books • Frederick Warne & Co • G.P. Putnam's Sons • G.P. Putnam's Sons Books for Young Readers • Grosset & Dunlap • HP Books • Jove Books • Ladybird Books • New American Library • Obsidian • Onyx • Roc Books • Signet Books • Signet Classics • Signet Eclipse • Topaz • Pamela Dorman Books • Pelican Books • Penguin Books • Penguin Classics • Penguin Press • Perigee Books • Philomel Books • Plume • Portfolio • Prentice Hall Press • Price Stern Sloan • Puffin Books • Razorbill • Riverhead Books • Sentinel HC • Speak • Jeremy P. Tarcher • TarcherPerigee • Viking Press • Viking Children's • Ballantine Books • Bantam Books • Delacorte Press • Del Rey Books • The Dial Press • Modern Library • Random House • Random House Value • Spiegel & Grau • Alibi, Flirt, Hydra, and Loveswept • Random House Children's Books • Alfred A. Knopf Books for Young Readers • Crown Books for Young Readers • Random House Books for Young Readers • Little Golden Books • Schwartz and Wade • Wendy Lamb Books • Ember • Bluefire • Dragonfly • Yearling Books • Laurel-Leaf • The Princeton Review • Sylvan Learning

Each and every single one of those publishers/imprints is owned by Penguin Random House. Some are owned only in part, with the other part owner being Pearson PLC.

Point being that that isn't even the complete list of the houses/imprints they own.

While that in and of itself isn't awful, it means they've successfully gained enough power to have some serious weight behind them in most markets. I work for a publisher in Canada, and there's honestly only a couple dozen independent publishers in the country that are of any notable size. The rest are owned, in some way, shape, or form, by Penguin Random House.

It has crippled Canadian literature. Not single-handedly, mind you, but they happily push titles they view as being more profitable, and they seldom see Canadian stories as being profitable. There's a false understanding in Canada that the reason there aren't more Canadian books in stores that either A: Canadians don't write good stories, B: Canadian books tend to be boring, and/or C: there just aren't that many Canadian books.

Thing is, though, as an independent publisher why risk tens of thousands of dollars putting out a flashy YA title or an exciting thriller or a grand adventure novel when you know the multinationals have complete control over that market? Better to put out a regional cookbook, or a non-fiction book about Alberta politics, or another biography of some Canadian figure; after all, the multinationals would see those things as being too niche or unexciting to bother with, so those are the markets you won't lose money in.

There's a mountain of Canadian writers out there with great stories to tell. Unless your name is Attwood or Coupland good luck getting anything someone younger than your grandmother would want to read published though. God forbid you set a story in Canada.

There are exceptions, of course. The multinationals aren't evil, but they're the Walmart of book publishing. Any market they enter shrivels in their shadow, and we're left with less and less media actually relevant to our country. The same thing happened over the past couple decades with Canadian television and film. Neither could survive competing against the mega-corporations of America. They're both circling the drain, and Canadian publishing isn't far behind.

TL;DR: I'm on a soapbox here so I apologize, but seeing the praise elsewhere in this thread for Penguin and other publishers like it... I had to say something. Penguin Random House, and other multinationals like it, control vast amounts of any market they decide to enter. In Canada their grip on the market is in large part responsible for a crippling of Canadian literature, evaporation of Canadian content from Canadian schools and libraries, and lands us with the existing stigma that Canadian books are too boring or too niche for widespread appeal.