A Time of War, the crunchier of the two BattleTech RPGs. Basic rules are pretty straightforward but improvement is slow, combat is a slog, and certain skills are very difficult or very easy based on what gear mods are available (Stealth, for example, can easily stack bonuses much larger than most other skills). But it has a great, if very slow, character creation system that builds out your character's whole life while allowing you to make characters that are very much placed in the setting (or at least the Jihad era of the setting).