Mine is easy to list. A serious noticeable regression in the debugger.
http://feedback.xojo.com/case/59520 The debugger simply doesnt work as expected and the more I work the more infuriated I get that this bug has existed for over 2 years now
Voting for it hasnt gotten it fixed (its #11 on their top user requests). And there are other similar bugs that seem related (like step not behaving a expected (http://feedback.xojo.com/case/68263)
Whats YOUR most important bug report for Xojo to fix ?
Bug - not feature request