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I have no favorite OS right now, I have problems with all the OSes I run. Enough of a problem where I can't consider any my favorite. Right now I run these OSes on a regular basis
1) Windows XP SP2
2) OS X 10.4.8
3) Open Suse 10.1
4) Open Suse 10.2
5) SLED 10
6) Kubuntu 6.10
I run a few knoppix distros but I consider those to be more of a tool for work rather than an actual user operating system. I wouldn't do day to day work on the knoppix builds, but one I do like is BackTrack.
Out of all of those OSes I really do not have a favorite because once I start to like one the developers do something stupid or something breaks or something changes I do not like.
I also run Vista on one machine at work and am not really impressed with it at all, and I don't run it for any other reason than to test it out so I wouldn't consider it an OS I use on a day to day basis.
I have (including work and home):
Gaming PC running XP Pro
Macbook Pro running OSX Tiger, kubuntu, windows xp, backtrack
HP NC4200 12" - Running Open Suse 10.2
HP DC150 - Running Open SuSe 10.1 and SLED 10 It also has Windows XP Pro SP2
G4 Dual 500 - OS X 10.4 OS X server (test server)
G4 MDD - OS X 10.4 and OS X server (another test machine)
Building a new gaming rig - it will run xp pro and possibly vista if I get a DX10 video card (i have two legal copies of vista ultimate)
HP DC150 - Open Suse 10.1 + DRBL box running clonezilla (imaging server that I built in our shop to image machines for deployment)
I also have a half way built MAME box, I just need to save up some money and actually go buy a house so I will have room to build it. Right now one really wouldn't fit in my apartment.