I have a huge collection of mp3s and I tried several (free) taggers. Eventually I decided to buy Tag&Rename since - for me - it is without any doubt the easiest, most intuitive program to work with. It supports getting the tags and cover art from different databases. It integrates into windows explorer.

The best for tag manipulation programme for any size collection! Supports lots of files and has come a long way since I started using it.

Found it much better than mp3tag but I didn't use it for long as it didn't work the way I wanted it to.

Updates tend to have good new features that make it better as it goes along. Last major update was album artwork view and folder artwork view and corresponding automated tasks for adding artwork to files and vice-versa.