Big Spaceport is a collection of levels designed from scratch that are meant for Doom 2 veterans. They are very large, and far more challenging.
Play "Hey Not Too Rough" for less of a challenge, it lies somewhere between "Hurt Me Plenty" and "Ultra Violence" in the original game. For most players, HMP should provide plenty of action for your first time with these levels; the item placement and most count are designed to be fair. UV is for the second time through. You are expected to know the level layouts and at least half of the secrets to be able to succeed. While 100% kills are possible, you must be very conservative with your resources and remember where you left supplies behind.