I couldn't possibly say off the top of my head.
But I think the most influential on me, would probably be Guns Of Tanith by Dan Abnett.
I read a lot when I was a child, but I think that was what started me on my actual road to becoming a fan of Sci-Fi, and led to me still freelancing book reviews to this day.
It's in the Gaunt's Ghosts series, it's actually the fifth one, from Games Workshop. I still read White Dwarf at the time and read a gripping passage of it they printed there, and went and bought the book.
It's got brutal violence, good one-liners, a murder mystery, political intrigue, espionage, I still consider it the best book in the whole series frankly.