Freeze timers would be good. Oxygen would be a minor help. Infinite ammo could be helpful, but once money is taken care of, ammo isn't too difficult to recharge. Otherwise, never going unconscious would be good too. Couple that with an explosive radius cheat, and one concussive grenade could wipe a floor... The Capture timer is my biggest annoyance, though.
Edit: The Overpriced Mystery Crate could cost nothing......... rng for that could be modified...
Edit2: Infinite Sidewinder teleport range, and it ignores pathing... Really, there are so many cheats that would change the game.
Edit 3: Money is stored as a double. It's very easy to AOB Inject.
Edit 4: Hot swap items in inventory for whatever we want, using a hotkey... I realize these are all very involved, and they're not all necessary, but it would be helpful for the missions that force your inventory to start a certain way.
Edit 5billion: Ammo is stored as a double too. If you do manage to get armor piercing ammo, it's very easy to find the correct code to adjust it to a larger number. I had a list of results that were all displaying ammo, but I went down the list and changed them manually until the ammo in-game changed. Then I knew I found the right one.