mikebeam
What would be your opinion of the best... I'm not interested in minimal learning curve - I'm fine with a big learning curve and lots of practice... (I understand that's very subjective. Just curious)
At this point, Ozone would be the best for you. It contains everything required for mastering, including a mastering limiter.
The next choice would be the T-racks mastering suite. It also contains everything you need including a mastering Limiter.
If you only want a mastering limiter, get one of the free ones mentioned, prefer: Limiter #6.
The best non-free are FabFilter, Voxengo, IK Multimedia.
The internet is full of video resources on mastering. Watch as many videos as you can. Groove 3 also has a good series on mastering. Watch the videos then try applying these techniques to your recordings.