Unexpected DC hits?

[iron chimp]

Watchmen and sandman are obvious ones.

[Lee Stone]

The Wolfman/Perez Titans.
I believe they found out around the sixth issue that they were outselling most of Marvel and were surprised.

Hitman.
Part of the 'DC and Marvel freak out over Image doing superheroes and try to trademark every edgy name they can think of before Image does', also known as 'the 1993 Annuals'. Hitman was DC's only thing to really last out of that.

The JSA.
When DC gained the license to do the Archie heroes in the early '90s, they had some lead time to fill while waiting for things to get started. So, they took the pool of creators and made a 'throwaway' JSA mini-series that stood alone and wasn't dependent on continuity.
Reception was so good that it sparked a revival of the team, despite that not being the purpose of the book.

JLI.
The longer it ran, the more haters thought 'no one could like this', but it just couldn't stop.
Writers and editors were forbidding their characters from joining, but the JLI defied them by creating stars out of the ones they were given.
It was the first time since All-Star Squadron that a Justice League title had spin-offs launch out of it, with 1989 and 1990 being its biggest years.
The title became one of DC's top titles, only dropping down a couple years after Batman mania started again and the formula began getting tired.
Led to Blue Beetle, Booster Gold, Guy Gardner, Fire and Ice becoming fan-favorites and solidified J'onn J'onzz as the heart/soul of the Justice League.

Lobo.
Minor villain in a minor comic sold only at comic shops when comic shops weren't the norm.
Suddenly he's everywhere.

Harley Quinn.
Started out as not even a comic character.
Took over the DCU when she transitioned over.

[Electricmastro]

Animal Man.

[Earth-2 Refugee]

I'd like to think the New Superman and Justice League of China have a cult following, but that fanbase seems to have died since the series ended. Otherwise it would be Injustice, which I expected to be crap like the games but turned out to be an amazing story (especially Injustice 2 since it was only Tom Taylor) and also DCeased (also Tom Taylor). I think his work has shown to be consistently cult classic stories, but I'm also a relatively new reader (started reading comics avidly with Earth-2 in 2012).

[9th.]

...Modern Aquaman?

Yeah Aquaman has came a loooooooong way, people used to automatically **** on him no matter how awesome he was.

[pynchon1]

Maybe Flintstones from a couple of years ago. The buzz around it at my LCS grew exponentially from where it seemingly started with indifference. I thought the same kind of phenomenon happened in the New52 with Frankenstein and Dial H, although the latter had a somewhat well-known author outside of comics attached to it.

[married guy]

Hitman immediately sprung to mind.
In the same vein, I remember when Preacher started to REALLY explode too.
Around issue 5 EVERYONE was raving about the new Vertigo title that you HAD to read.