Calum, if Solaris isn't unix, Unix pretty much doesn't exist (since nothing else will qualify). So it is.
And besides, what is that chart you linked based on? Usually those charts are based on code usage, but that means Linux is off the chart (or seperate anyway), but if not that, there is no real criteria. Linux wasn't really based upon Minix, you know, it was based on a lot of systems (Minix mostly, but far from entirely).
To the point, of the systems I used I like Linux best, with Solaris a close second (I don't have too much experience with Solaris though). Windows NT is third with a whole lot of distance, followed by Win9x/DOS. Those are the only OS-es I used really (I'm a lameass), so any others should be considered unrated.