As a simple Guide A's are Better than B's are better than C's, but only for the same size module.
All of the module slots also have a number assigned to them, this being the rating, so for instance if you have a E3 Shield Module, then a D3 down to A3 will be better. However an A2 or A1 won't be better.
Mostly you'll want to the same sized module rating, (1 small, 2 medium, 3 Large etc) whilst improving the module class from an E to an A eventually.
Note: B's tend to be the same as A's but B's tend to be heavier.