The Original Terminator is the best. The pacing is fast, it has a great story, it is a very dark movie, and Arnold is cast to perfection as the bad guy.
I do give props to T2. I still would rate it behind the original, but it is a great sequel, even one of the best ever. The action is great, the effects are awesome, though it does have a lull in the middle of the movie, but it is always hard to top a classic movie when making a sequel.
T3 is irrelevant to me. It is a descent action flick, which is enjoyable to watch, but does not compare to T1 or T2. There is no character development, it has a repetitive action chase scene taken from T2, and Arnold goes to a bar to get his clothes at the starting of the movie again! Come on writers, get some originality!