Terminator 1 was more impressive to me considering the budget. One of my favorite scene is Reese's dream sequence of the future where a non-Arnold Terminator got through the resistance's checkpoint and shoot up their hideout.
Terminator 2 of course gets the nod for everyone's favorite action film. It had a much higher budget, improved sfx, and improved sound (the original was in mono given the budget constraint). One of the best opening sequence of any movie..period. Better plot? Maybe, but it's a continuation of the 1st movie. It's more of an elaboration of the 1st. Better acting? With Edward Furlong's acting, I'd have to say no. Also the tone of the movie seems less serious than the 1st with a bit of humor added in as if appealing to the MTV crowd. For me that's a minus. Overall it is the best Terminator movie.
Whoever said Terminator 3 was better than the first, you might as well shout out "I have really really bad taste in movies." If T3 was a separate movie from the Terminator time line I'd be more forgiving. As it is, T3 is the sh!ttiest addition to a trilogy ever. Whoever made this travesty spat in the face of the first two films. Didn't even pay heed to the underlying central theme the 1st two films emphasize, that the future is not set: what John Conner made Reese recite to Sarah, what Sarah was engraving on the picnic table, what young John Conner keeps reciting in the 2nd film "there is no fate but what we make for ourselves." I'm not even going into the whole cheesiness about the role of the virus and the internet.