Robert Heinlein’s Starship Troopers and Joe Haldeman’s similar but Vietnam-tinged miltary sf novel, The Forever War, are both well worth a read.
Yes, the Old Man’s War series is excellent. There’ve been five novels, a novella and two short stories so far; they hold together very well.
FWIW, Consider Phelbas is probably one of the worst of the Culture novels. (Still better than The Hydrogen Sonata, or most of Matter though.) Which isn’t to say it’s bad. It is probably one of his more space opera-y novels though. And it’s not short. I liked it, but I’m posting to say that if you didn’t think it great, the series gets better. The Culture novels don’t really have anything to do with each other, so read them in what ever order you want.
Another space opera I constantly recommend here, but it’s not military science fiction at all, is Alastair Reynolds’s Revelation Space universe. More Gothic horror than typical science fiction, but it’s one of the few series that takes Clarke’s observation about ‘sufficiently advanced technology is magic’ to heart. Weirdly, only the first half of the official trilogy is any good, but the other works set in the universe are great.
EDIT: Oh, and take a gander at Stross’s Neptune’s Brood. Neat if you like an economics discussion with your transhumanism.
I also love Old Man’s War. John Scalzi is one of my favorite sci-fi authors. I’m enjoying his new series as well.
Me, too (The Collapsing Empire etc.) but there’s not much military sf in there.