I've been playing in or running Kill Sector for a couple of years now. I think it's a fantastic sandbox system which emphasises tactics and unpredictable, very deadly combat. It fits one-shots best, but longer stories with character growth over time is also feasible. Creating a first character can seem overwhelming as it's a point-buy system with over 1000 options, but that's also what's cool about it. Although some setting info and modules exist, it's really more of a universal platform for character creation. I'm using it as a basis for developing fandom and genre games; I simply cut away the parts that I don't need, and everything that remains works. Tyson and Noah have been amazing about in how they frequently rebalance functions, add new features, and support the community -- all this for the whopping price of free.