For the easiest thing in town, I'd say "Atlassian Jira" https://www.atlassian.com/opensource/overview
I am using Fogbugz and JIRA on several client projects and JIRA definitely has more traction and engagement. But, migrating may be less painful than before: https://confluence.atlassian.com/display/JIRA/Importing+Data+from+FogBugz+for+Your+Server https://www.atlassian.com/software/jira/importer-migrations#!jira-fogbugz Phil
