Hate me if you want, but I think the order should be:
1. Terminator
2. Terminator: Salvation
3. Terminator 2
4. The Sarah Connor Chronicles
5. Terminator 5
6. Terminator 3
And here's why. First of all, the whole concept of 'Terminator' was killer robots from the future. I don't care much about Arnold as an actor, but he's the perfect Terminator. Many people don't understand why I hate the new Friday 13th movie, and especially that version of Jason. Jason never talks, so isn't it enough to have someone put on a mask to be Jason? Wrong. Jason never speaks with his mouth, but he certainly speaks with his body language. Compare Kane Hodder as Jason vs anyone else as Jason, and you'll see it right away. Kane Hodder is the master of being scary without actually saying anything. The same goes for Arnold. The Terminator never really speaks much, and when it does, it's only short lines. ("Sarah Connor?" "Yes?" Boooom!)
But Arnold also speaks with his body language a lot during the movies. Take the scene in the nightclub, for instance. There's a lot of stuff going on, but the Terminator clearly doesn't give a crap. It's there to kill Sarah Connor, and that's it. Everything and everyone else is unimportant.
Then we have Terminator 2, and Arnold is suddenly the good guy? And although he's a killer robot from the future, he has no problem avoiding killing anyone at all? That part just felt wrong. And what about the Sarah Connor / John Connor / Terminator relationship? The Terminator is supposed to be a killer robot, but it acts more like John Connor's father. On top of it, the movie feels too similar to the first one. (and btw, it's been ages since I saw the first one, but didn't Reese have the line "Come with me if you want to live?") Sure it's more of it, but it's more of the same.
That's why the second place goes to Terminator Salvation on my list. Salvation has a lot of problems, but it feels like it belongs in the Terminator-universe and is more "grown up" than the last two, and it feels very different than the first three. We finally have a new story, and not just the old one on repeat again. Sure it has problems, but so does all the other movies, including the first one. But unlike the third and parts of the second, it's very enjoyable. I had a lot of fun watching it.
My fourth goes to The Sarah Connor Chronicles. True, that show did have a lot of problems, and since it's based entirely in the present, it was bound to get boring quickly if all they did was have Sarah and John Connor on the run with a terminator after them. I think they solved it perfectly, but even so, it was going to get boring soon. But until then, it was very interesting with lots of action and terminators. Even terminators sent back to the present on different missions other than the usual killings.
Then we have Terminator 5. Very little is known about it, other than it's in development. But then again Salvation was supposed to be the first in a new trilogy, so I guess that's not a shocker. But although we know nothing about it yet, it can't possibly be worse than the crapfest called Terminator 3. T3 was so bad even the tv-series ignored it.