How do you toggle between windows in Parallels?

If you want to switch between windows in any Mac application, it's command-`. Between applications would be command-tab (not control-tab). If you're talking about Windows applications running within Parallels, that's a Windows question that would need to be asked on Microsoft's forums.

Alt(option)-Tab, and sometimes Alt(option)-`[the key with the ~, above tab/under esc.] works to switch between windows within a Application...

Not here. Not on any Mac I've ever seen. I think you're confusing option and command.

Thank you kind sir!! Been wondering for a while how to toggle between windows within an app.. BRILLIANT. You have just increased my productivity by several minutes per day..!! cheers