T2
TS
T-I was born in '81 so it was a few years after its release that I saw the first one but T2 was right behind it and for me and thats the one that resonated with me.
For me, T1 and T2 are about equal, a rarity in movies and their sequels. Both are complete stories onto themselves. T3 pushed it and Salvation disappointed the most.
First two are classics. T3, well i rather enjoyed it. TS, was flat. So much potential was wasted. Still enjoyed it though. Totally agree with the poster who mentioned the music as being a very key element of these films. The music wasn't used enough in T3, TS, hence crippling those two.