Dear ttuhd
An extraordinarily difficult question to answer. It is so personal and depends on your needs, interests etc.
So here's a few (mainly free) apps which I use a lot.
VLC already mentioned. Handbrake (which works in conjunction with VLC although they are totally separate apps), is for interrogating, legally copying DVDs.
1Password is widely used by Mac folk as a very secure PW generator, PW and Log In Manager and much, much more.
Flip4Mac, free, allows you to view imagery, videos created by Windows Media Player on your Mac.
OnyX is a free Utility that tests your Mac for faults, does lots more and is a very handy Maintenance & Troubleshooting app.
I suggest that before you download the free or purchased apps, you read up on them. Macworld has numerous reviews of these and similar products. Senior Editors in Macworld regularly offer their "can't live without" apps.
They also offer their tweaks and settings for the Mac on first purchase - and on it goes.
Explore Macworld and Mac Forums for previous discussions and suggestions.
In the end, it's down to you.
Best I can offer, sorry.
Ian