http://i.imgur.com/IXJ8qSn.png
This is something from this thread: http://teamfortress.tv/forum/thread/1189-hook-me-up-with-this-crosshair that is really good, but as you might tell just from looking at it, the shape becomes very blocky at low resolutions (1280 * 720)
http://i.imgur.com/eh8Pgez.png
If this had an outline that worked at this resolution that is what I would always use