I thought I would update my opinions as I have now seen the release of Windows 7 (Release of the hounds) and have seen what it looks like. My opinion. Garbage. Too much like the Mac OS X Interface where you have gadgets and widgets and all of that other junkware embedded into the OS. The fact that I can't see my programs on the task bar and that I have to hover over these mystery icons to find out where they are makes Windows 7 terribly hard to use in comparison to XP and 98 SE. The only reason why I ever have had Windows 7 installed is for testing in a virtual box and dual booting with Ubuntu so I get support from a technician for my expansive hardware (i7 8 core 3.2GHz & 12GB ram).
Ubuntu is a nice, clean os -- but there isn't much you can do with it. There aren't any games written for it and AFAICT its only useful for using as a web and/or database server. Compare the amount of commercial software available for MS-Windows with the amount available to *nix. MS-Windows blows *nix out of the water.
Installing new programs on *nix is a nightmare. With MS-Windows all you do is download and click the install button. Done. With *nix you have to worry about getting all the packages, and often you have to compile the programs yourself. I understand the reason why you have to compile the programs yourself, but still ...
The only really nice thing I have found about Ubuntu and other *nix distributions is the price -- they are normally free. But, you get what you pay for.
if you r a regular/general/normal computer user, then windows 7 is probably the best because of the amount of compatible software on the market.
secondly, windows 7 has better GUI design than ubuntu(the best linux extro) and mac,
for instance, it is more natural to have the task bar at the bottom than at the top.
For mac and linux, having 1 bar at top and 1 bar at bottom are not really practical. though mac is prettier and fun to use, but it is just not efficient, and ubuntu is just ugly and too technical for normal users.
that having said, if you r a professional programmer, then linux is probably best for your professional use, because you can modify it to suit your need.