I would never buy a horse from the Pilot lines because they tend to be hot headed and difficult, or SJ lines like Cassini, because I am not a jumper.
Horses from lines that generally have a nice temperament are well sort for, like Rubenstein. Horses from Donnerhall lines are in high demand for their movement, size and temperament. Fishermen's Friend, as Kayty has said, is a very trainable temperament.
Not to say that I would disregard anything, and I don't go out saying I am looking for this line, but if I see a line in there that is problematic, I won't touch it. I'm not a good enough rider. If the lines have good, well mannered horses, I'm more likely to pick it.
And if I'm buying a mare, which I did, it better have **** good lines... stereotyping, I know, and not had a problem with mine but with some- the mood swings!