My first reaction, whenever this question pops up on the forum, is (of course) Starship Troopers.
But that would be dishonest.
I must confess that I read Ender's Game, cover to cover, over one schoolday. Unfortunately, Orson Card doesn't have the experience to write a really convincing military narrative. But Pournelle's The Mote In God's Eye is everything that a space dreadnought book ought to be, right down to the grizzled old petty officers and the inexperienced midshipmen.

Oh, forgot to add- 1632 gets a big ol' thumbs-up from my corner. You can get it for free from Baen's electronic library and I absolutely loved it. Yes, I'm sure it's riddled with inaccuracies, but it told a glorious story and had the guts to be completely partisan for the American ideal. There's precious little of that stuff these days- it's unfashionable to be a patriot, even worse to be pro-American, and I'm glad to see Eric Flint was willing to wave the flag a little. It's not all McDonalds and Abu Gharib, you know...

I suppose I should give Haldeman's Forever War a plug. It serves as a good contrast to Starship Troopers, even if it is in good part a rumination on the Vietnam experience. It isn't particularly glorious. If you want action, blood, and bravery you might want to give David Drake's Redliners a look. (Available for free from Baen's online library, again.) Hammer's Slammers is recommended by everyone with good reason, being in the same vein. There is also a fun trilogy by David Weber which begins with Mutineer's Moon. The first book is good enough on its own, and the sequels are full of nail-biting clashes between opposing battlefleets.
The Stars Must Wait by Keith Laumer is a fun little novel in a sort of post-apocalypse vein. (It has a Bolo. You could do worse.)
Finally, if you didn't care for the romantic subplot in God's Eye you might give Honor Harrington a look. To date I've only read the first book of the series but enjoyed it.
One final recommendation- Apocalypse Troll by David Weber, again. Happy reading.

  I have read several books since my last posting and have some new recommendations.

 Based upon numerous recommendations here I read Card's Ender's Game and went on to read most of his work.  Of Card's books his best was Ender's Game and Ender's Shadow (same story told through another characters perspective), the rest of the Ender's series was enjoyable but did not rise to the level of those two works.  I also enjoyed Card's Alvin Maker series; a type of alternative history set roughly in the early half of 19th century America where it is common for citizens to have supernatural 'knacks' and hexes work. 

 As for science fiction with a military theme I most recently read and most highly recommend Jim Butcher's Codex Alera series.  Jim Butcher is most famous for authoring the Dresden Files series which was picked up for a season on the Sci Fi channel.

 The Codex series is set in an alternative world with different types of humanoid citizens.  The humans in the dominant nation, depicted as a post Romanesque medival feudal society, utilize a type of magic by summoning the abilities of elementals called 'furies' with the nobles or 'lords' having more powerful control and access to furies.  Other societies and creatures in that world have their own supernatural abilities and are in a near constant state of war.