This is perhaps not the most gut wrenching decision in the series, but still worth noting regardless. The penultimate decision made in the original Mass Effect is still a powerful moment for Shepard. In the climactic final battle between the geth and the Alliance, Shepard is forced to make an order that will decide the fates of the entire council (as well as one of the most powerful ships in the galaxy) and a whole lot of lives. Your dilemma isn't too complicated. You can order the Alliance Fleet to save the Destiny Ascension and the council along with it, or hold back and leave the Citadel Council to die. Important to consider is the sheer size and power of the Ascension, which can be of use to the player in the third game. However, doing so comes at the cost of many human lives. Aboard the Council's ship are a crew of nearly 10,000, whereas the losses sustained when saving the ship include twenty turian cruisers (that's roughly 6,000 crew members) and eight Alliance cruisers. Either way, there's no ideal solution, so you'll just have to make a choice and live with it.