My favorite writer would have to be David Weber. A lot of what he writes is military science fiction, with a focus on the tactics etc of battles and a fair amount of politics. I love the series he wrote with John Ringo--first book is "March Upcountry" and there are four. (I believe I saw that more could come in the Empire of Man series, which is very exciting!) It follows the youngest son of the empress as he goes from "zero to hero" through a really, really tough situation. I don't want to give anything away so that's the very short version!
His Safehold series is also really good--it is a mix of future and past and religion, after humanity's last colony becomes a very religious colony to prevent technology. Not everyone agreed with this plan, and so someone is working to bring technology back. The battle focus in this series is naval ships, but actually on the water unlike the Empire of Man and his Honor Harrington series. (Again, I don't want to give anything away, but I am a big fan of this series.) I think the sixth book of this series just came out in June or July? I'm behind--haven't gotten my hands on it yet!
The Honor Harrington is the series David Weber is most known for, I would say; it is at least 11 books long and follows the female character Honor Harrington. Again, this is military science fiction, with a heavy dose of politics between space civilizations.
He has a fantasy trilogy as well, not just science fiction! I really enjoy the War God's Own series. The main character becomes a reluctant champion of the war god; he is from the race that the other 4 fear and hate so it is even harder to be a champion.
Can anyone recommend another military science fiction author or book/series? (I'm working my way through this thread to start a list of authors to look at, but it is so long it is going to take some time!)