Public cheats are detected in few hours, in some cases few days. Its a bit more difficult with private cheats although PB has some heuristic checks to catch these ( also keep in mind that PB detection can be in so called silent mode ). As soon as pbsvc is fully operational , it will much more difficult to bypass PB. PB is atm the most advanced ac out there.
Easiest is imo FEAR.It's not a difficult to make a cheat for game based on Q3 engine which was developed in 1999. Keep in mind that Id software released it's source code under GNU/GPL, thus you almost don't need any debugging/disassembling skills to find out what you need to know to make a cheat.