If we want the license to be a) as permissive as possible and b) legally watertight, there a basically three licenses to choose from (dismissing for now many, many lesser known):
1. The ISC License 2. The BSD License (3- or 2-clause) 3. The MIT/Expat/X11 license
All three are GPL compatible and OSI, FSF and Copyfree approved and should withstand legal actions. The ISC license is basically a modernized version of the 2-clause BSD license and is the preferred license of the OpenBSD project since 2003.