1. T-X (Terminator 3: Rise Of The Machines)
This is a really obvious one I'll admit. The most advanced terminator is noticeably worse than the predecessor and is beaten in a manner that would have been easily avoided by the T-1000. This villain is even worse in the hand to hand combat scenes than the T-1000 with Arnie actually holding own more than really makes sense for the story. A minor point maybe but the terminator in this film is a T-850, a slightly upgraded model from the previous two films, but this should have been a very one-sided battle. Terminator: Salvation didn't fall into the trap of trying to recreate the first two films and that was where Terminator 3 failed, and to have Terminator Salvation as an improvement is not good. Terminator 3 failed to follow its predecessor in both the main villain and the plot, and the T-X should by definition be the ultimate killing machine. Unfortunately the T-1000 had that distinction and what we had here was a supermodel in a leather bodysuit who never seemed the present any real threat and this was as much to do with the laziness of the plot as the character herself.
This model of terminator had the "advantage" of built-in weapons which Arnie and friends promptly sabotaged. The T-1000 was very close to indestructible and was resourceful enough to just attain weapons and in any event a hard pinch would have killed off John Connor once the T-850 had been incapacitated. In short, having built-in super-weapons is like buying an assault rifle to kill a fly rather than a fly-swatter and then shooting yourself in the foot (profound analogy being made here), it's obviously a flawed approach to say the least. As underwhelming as the T-X is in ability and effectiveness, she/it also lacked the menace of the antagonists in the previous two films as well which didn't help one bit.