Well, I play a LOT of games. But overall, I think the absolute best "military" game I've played has to be Red Orchestra: Ostfront 41-45. Extreme realism, authentic equipment and maps, and incredibly intense and battles. They also nailed the sound design perfectly, which really contributes to the atmosphere and making you believe you're actually a Russian rushing a machinegun position over the Konigsplatz.
Aside from that, I like the first Ghost Recon, but not any of the other games in the series.
Also, Rainbow Six 3: Raven Shield is one of my favorite games, though it's more about counter-terrorism than the military. It has a huge selection of real weapons and it was the last game in the series in which the enemies are really trying to kill you and not hiding and spraying. This is probably because it was the last game in the series developed by Red Storm before the French took over development and ruined Tom Clancy's credibility to the gaming community (well, technically, it was Splinter Cell that did this first).