Book reader community.
It depends on what about grimdark appeals to you.
For ultraviolence: Acts of Caine by Matt Stover, especially the first book. Though this is maybe sci-fi?
For snobbery hoity toity book club cred: Shadow of the Torturer by Gene Wolfe
For ultimate chunk: The Malazan Cycle by Steven Erikson. I recommend reading with a wiki open. It's worth the struggle, but it IS a struggle to get started.
My word, ‘ultimate chunk’ appears to have been an understatement! Gonna have a go at at Gardens of the Moon - thanks :)
The Black Company may be something you'd be interested in.
Wow this book appears to divide opinion! The first review in that link absolutely slams it at length, but then others are claiming it to be the best thing ever written.
What’s your take? Is it really a love it or hate it kind of deal?
I read several books in the series. At first I thought the story was pretty interesting...but then it just kind of seemed to stall out, not go anywhere really, and I lost interest. It's one of those books where it seems like there is a lot of background that is going to be cool to finally learn, but I never found that point and I got a little tired of being honeydicked.
Personally I'd say read the first book. I've suggested it to several friends with mixed responses. I should note that I had the same experience both times I tried to read through Robert Jordan's Wheel of Time series. I made it until book 6 or 7 and just lost interest. That's only happened to me with a handful of series over the years.
tl;dr I get people that hate The Black Company. I don't get people who think it's the best thing since sliced bread. But the first book or two are interesting reads and I recommend trying them out.