Close call, but try Starship Troopers instead.
They are equally good. They just have different scopes. Starship Troopers focus is on why a soldier fights, while The Forever War focuses on what can happen to a soldier when fighting. They also reflect 2 different societies: one where the soldier is not only celebrated but serves as the core for society (ST), and one where the soldier is alienated and returning to a world they no longer recognize (TFW-this was also the experience for many Vietnam War vets). There are some interesting parallels in that in both cases the soldiers aren't dumb cannon fodder but are specifically selected for intelligence and physical ability. They both also touch on the impersonality of war. In ST the comparison of the cap launcher to a gun (capsules lined up like rounds, fed into a chamber, and shot out) is very apt as the soldiers are essentially aimed at a target, shot at the target like the living willing bullets that they are, and allowed to do what they do (ie, cause lots of targeted damage). In TFW we see soldiers that are used as cogs put into a machine that has a specific purpose, and when one cog breaks it is quickly discarded and another one gets pulled out of the box and stuck in its place. Again this is a direct response to Haldeman's experience in Vietnam, where most soldiers were drafted, placed into a combat unit with no one they knew, and when killed or wounded simply replaced with another draftee. The cold heartlessness of this process is reflected in the way that soldiers were selected and used by the military in TFW.
Of course, ST is the more "upbeat" of the two, and reflects their author's own view towards war: that war is a necessary evil (Heinlein) and that war is just plain evil and mostly unnecessary (Haldeman). Both books are excellent in their own rights, but it is perhaps telling to note that, according Wikipedia, Scalzi wrote the foreword for the most recent printings of TFW and that Heinlein called it "may be the best future war story I've ever read!". I am really looking forward to seeing how they adapt the film version. If they keep to the theme of the book it could be an excellent movie, kind of along the lines of a scifi Platoon. If I don't leave the movie depressed but slightly optimistic with humanity I will be disappointed.