Originally Posted by Enlightened Hand
It's all about what you are specifically doing. No single effect is useful in all situations, though in most situations it's useful to have a flexible equalizer. There are of course different kinds of EQ. Some are for careful correction, leaving little to no audible "fingerprints". Some are for sweetening and specifically have a flavor they impart. Others are somewhere between the two and some are very interesting in that they can be sweet and surgical depending on how you use them.
Equalization aside, whatever effect you need is best chosen based on what you're specifically dealing with, not necessarily genre of music; I find genres less useful for deciding on gear than the specific sound you're after, and it takes a while to know what kind of sound you need and what kind of sound certain pieces give you. If, for example, you find you want to control the dynamics of a vocal in such a way as to make it seem like there is nothing going on, yet the vocal just sits in the right place, then you might be looking for a compressor/limiter that didn't leave too much of a fingerprint on the sound but could handle a vocal's dynamic range. That's a genre independent decision for the most part.
He's got a toft for the eq, and is happy with internal compression. An eventide is the perfect effects, and will compliment both of the above.