Phew.....so hard to pick one favourite....
I'll have to go with Terminator II.
I went to see it in the theater when it came out....thats's ages ago (I'm starting to feel old just writing this)
I like the way that the grim future is portrayed. Machines taking over has and always will be a plausible theme.
And for that time, it had some pretty amazing special effects, especially the scenes with the T-1000 in it.