I think Terminator 1 is the worst of the 3 movies. It was probably seen as pretty groundbreaking when it came out, but it hasn't aged well at all.
T2 is the best one, but it's also pretty much the perfect action movie, so it's not fair to compare the others to it.
T3 had cool action, robots fighting each other, acting that wasn't so bad as to be painful to watch (though it certainly wasn't great), and took the series in a fairly interesting direction.
Also, apparently the Kate Brewster character is going to be in the next movie, so that pretty much confirms that it takes place in the T3 timeline, and any continuity from the Sarah Connor Chronicles will be disregarded, which is a bit disappointing, but understandable, I guess.
If nothing else, it will have lots of cool action, judging by the trailer.
Also, apparently Christian Bale said that he wouldn't do the movie unless he was satisfied that the script could be done as a stage production with no special effects, and still be a compelling story, which led to some pretty significant re-writes.
