I guess the system wich gives me the most problems would be my oldest Dreamcast its verry picky when it come to games but shes and old warrior I got it at a pawnshop and it was so nasty I had to take it apart and clean it from the inside out I also lubed the metal rails that the lens slides on it seemed to help tremendously
I have two DC but the other I bought new so it works great
also the most reliable and trouble free system ive ever owned has to be the Gamecube I guess IBM knows how to make a good system