Windows 7 is by far the best. It's not even a personal opinion. It's a statement of fact.
Depends what is the question you are answering to.
Many people seem to take the original question as "Which Windows version is technically the most advanced?". Well, duh, Win7 of course, being the newest and all.
I, on the other hand, answered the question as "Which Windows version were you the most happy with overall, in its time?", especially looking into it from a PC gamer's point of view.
As said, Win9x were a remarkable milestone for me in that they moved PC gaming ahead from DOS (no more strict HW compatibility requirements for graphics/audio, better game controllers, and better online gaming), yet at the same time they retained full backwards compatibility. I don't think there was even one DOS game I couldn't run on my Win9x machines (compared to that same system running only MS-DOS 6.x), even if it meant letting the game boot out of Win9x into real DOS mode.
On the other hand, Win2000 and WinXP caused some backwards compatibility issues with early Win9x games (let alone DOS games, but fortunately DOSBox came along much later, and there was VDMSound before too...), so moving to 2000/XP meant saying bye bye to some of your PC games. Unless you made a double-boot system, or kept old PC around. But at the same time, they certainly brought lots of great things to home users, like at last getting rid of the sh*tty FAT filesystem etc.
Pretty much the same with Win7 64bit. It's backwards compatibility with older games is pretty good overall, but problems with e.g. 16bit installers and updates on old Windows games, the well-known 256-color problems, not being able to select your MIDI devices anymore for DOSBox (Munt emulator) etc. Little retro-gaming annoyances like that here and there.
And also, I still dislike many features of Win7 Aero, the completely useless transparency sh*t, default colors which make reading even the digital clock hurt my eyes etc., which also lowers its standing a bit in my book.
But as said, if someone asked which Windows I would use as my primary Windows OS today, Win7 of course! Silly question, the older Windows versions don't have the latest IE or security features, and the official support from MS has already ended or near ending.
Even if I end up disliking Win8 and Metro, I'm probably forced to say "Win8" or "Win9" sometime in the future as well, if not for any other reason but MS eventually dropping support for Win7.