These are the apps that I consider a MUST have for my usage:
1.)Mvideo player - infinitely better than the stock video player and can display subtitles also comes with great folder management
2.) SMS backup and restore - to back up and restore my SMS inbox when I flash to a new ROM
3.)Opera Mobile - Something thats pretty much close to the functionality of a Safari browser, puts the stock andriod browser to shame!
Other apps already mentioned like G+, flashlight app, youtube app and ect!
Because everyone has a different level of standards, others might be just lower than average.