My list of must haves
Media apps (Audio & Video)
XLD - For CD ripping, format conversions (FLAC/AAC/MP3 etc)
Metadatics - For editing audio file metadata
VLC - Media player for videos
MPEG Streamclip - Video format conversions and editing
Quicktime 7. Yes, 7, not quicktime X.
MKVtoolnix - For remuxing, tagging MKV files
Utilities
iStat menus - real time hardware monitoring
Onyx - Once in awhile system maintenance. To be used sparingly and if you know what you are doing. (yes, finally someone with Onyx)
gfxCardstatus - To force using either integrated or discreet graphics card.
Little Snitch - Network monitoring and privacy
Backblaze - Online backup solution
A better finder rename - An excellent way to rename files on your comp.
Diskwarrior - If your hard drive goes wonky and this can't fix it, most likely nothing can (save a professional data recovery service).
I've not mentioned any of the standard apps that comes with your mac which are already "there".