=\ I really don't care, but I have a high screen resolution and themes that are from one end of the screen to the other really bother me. Depends on the person anyways.
Fixed, because some people have bigger resolutions and the forum looks horrible with all the width.
Fluid. I have all of this space, I want to use it. Then again, I don't care as much about how things look as long as they get the information I want across.
Depends on how small the fixed one is. I hate fixed themes that only use 1/4 of my screen.
I have the fixed on 1000 px and I look good on any resolution, same thing for my users.
Fluid..with all the widescreens nowadays you want to give the user the option of their own comfort zone.
The problem I have with fluid though, unless its a forum or something where there is lot's of content to display you often end up with lots of white space which doesn't look any good.
Fluid with a max and min width, that's my favorite one. Because you provide big resolutions a good width, but you can keep it usable. Reading through very long lines of text is not comfortable for most people, so full fluid would create huge lines on big resolutions. This is also good, because small resolutions can still see the theme in a nice way, but if they use really small windows, scrollbars will appear because some stuff would mess up in a too small width.