If someone had never seen a James Cameron film or even heard of him, which film would you recommend they'd see as his quintessential work?

For me it was a toss between Terminator and Aliens, I voted the latter.

Honestly, the one that jumped out to me was "The Abyss". It had the emotion from "Titanic", the alien factor from "Aliens", the apocalyptic element from "Terminator 2".

I voted T2, cos I've watched it oh so many times.
It's the one that I know the best out of the list.. Yep.

All great choices. T2 was definitely his biggest mark on the industry and was, of course, a fine film as well. I do agree with the reasons for chosing the Abyss as well.
My personal favourite is 'The Terminator' - I always find that greatness comes from directors that know how to make low budget work for them and that's exactly what Jim did on Terminator. It seems incredaible to me now that the budget was only 6 or so million dollars.

Was it Jim that chose Arnie for the role? ... if so then that makes it one of the masterfull pieces of casting ever.

Cameron originally envisioned the Terminator as a small unremarkable man, giving it the ability to blend in more easily. As a result, his first choice for the part was Lance Henriksen.
Schwarzenegger was offered the role of the human soldier Reese. However, they realized that he would be better suited as the Terminator, which as a result became large and muscular. Michael Biehn was also on the shortlist for the Terminator, and not the hero Kyle Reese.
Thinking Arnold would object to being the villain, Cameron decided to 'start an arguement' with him over a dinner so he would leave of his own free will.
Then, just as Jim wanted to 'ignite the fuse' Arnold blurted out that he was much more interested in 'Terminator' than 'Reese' and wanted the title role.
Jim thought, 'Thats a good idea...' and the rest is history.

Very interesting

Good old Arnie wanting the title role LOL

Well, apparently Arnold and Jim had to lobby hard to get Arnold the role as both Arnie's manager and agent thought it was a disasterous move that would ruin any chance he had to make a film career.