In terms of pure greatness, not action, I feel Platoon is clearly the best. It's a movie that even people who don't like war/actions movies probably like. It won the 1986 Academy Award for best picture.
It's just a great movie about good vs evil, naivete vs cynicism, innocence vs guilt, rich vs poor, asking complex questions about war and the gray areas surrounding it. There are parallels to Christ and Judas throughout.
But, yeah, if I just want more mindless entertainment (and nothing wrong with that), I'll watch FMJ. And I'm not saying FMJ is totally mindless (Kubrick is only rivaled by Marty), just not as deep as Platoon, IMO.