Everybody here have many good reasons to use GtkRadiant, or WorldCraft, but it I think the most intuitive one seems to be QuArK6.4.. see
http://sourceforge.net/projects/quark/
I'm currently mapping with this tool, and never it crashed... I'm also using aguirRe TxQBSP, Rvis, and Light tool that can be found at
http://user.tninet.se/~xir870k/
I hope it will help you into your editor choice...