Maybe the PDP-1? Or the Tera MTA? Or the Transputer? Or the CDC 6600? Or VisiCalc? Or the 6502? Or Intel's first X25-M SSD? Or Gmail?
Preemptive multitasking by itself isn't very interesting. It's useful if you're running a real-time control task (in which case, on MS-DOS, you would hook an interrupt, and fuck the lack of OS) or if you have memory protection, which the Amiga didn't. That's why the mainstream OSes didn't have it for another decade: because it didn't matter. (And concurrency control is much simpler without it.)
The Amiga had lots of other awesome stuff in the late 1980s, stuff that (as others pointed out) still isn't mainstream. But preemptive multitasking without an MMU is mostly a waste of time.
Gmail was so much better than the competition — Hotmail and Rocketmail — that Microsoft thought it was an April Fool's joke and advertised that new Hotmail users would get something like a terabyte each.
It's disappointing that you deleted your post, because I hadn't heard of the http://en.wikipedia.org/wiki/CDC_1604 before. The way that I heard the PDP-1 was significant was that it was something like 100 times the speed of any machine within a factor of 10 in price. But I can't source that in any detail. Is it possible that that statement was true but only until the 1604 came out in the same year?
I will say that I thought Yahoo was almost strictly better at Gmail launch, except one thing... storage. And Yahoo was actually reducing the size if I recall correctly.
I do use gmail now, although I still find the UI, on the web at least, to not be to my liking.
The Amiga had preemptive multitasking in 1984. Had VideoScape 3D in 1987.
It's hard to say that they were building computers for today, when they're machines were doing things the mainstream OSes (Mac and Windows) wouldn't do for a decade.
It would be like introducing the iPhone in 2000. And then doing nothing for 10 years.
I honestly can't think of another piece of computing technology that was so far ahead of the field when released. And yet virtually all of it squandered.