I'll always feel that the original is the best in the series, for that unique early-80s neon-soaked atmosphere (love that particular grain of film too), but I do think T2 had everything, really - utterly exhilirating action sequences, mindblowing sfx (for the time), lots of unforgettably chilling & brutal violence/creepy horror without OTT gore, an expertly planned out & paced plot with some quite deep themes transcending your standard sci-fi fare (feminism in a pretty fucked up, hard male-dominated world; fatherhood), a strong villain & an atypical 'hero', and even some tongue-in-cheek humour delivered with a knowing wink.
It's one of those sequels that fulfilled every criteria you could possibly ask for. It was never gona be the same moody lo-fi type of film as the first one again, that wouldn't have been workable with the blockbuster budget and need for big box office returns. Like Aliens, it does everything it's trying to do so well it's hard to dislike, no matter how it stands up to the original.
Of course, they then totally fucked up what could have been very interesting chapters in the big story with T3 (nice ending, but the casting can fuck off, and the 'terminatrix' is the dumbest bag of shit idea in the entire series) and Salvation (what you get when you hire a bunch of hacks).