I've used the MC series, the HF series, and the ER4S (older gen than the current ER4XR/SR). Going by the older models, the ER4 is quite a bit clearer. If you're into careful listening (as against playing music when in a gym, for instance), you can make out more details and texture with the ER series. The HF series are closer to the ER4, but even those have less details. By details, I don't mean the etched detail which you get when a transducer reproduces highs a bit strongly -- that's the kind which gives you listening fatigue after some time. I'm referring to details in the texture of, say, voices, which emerge from the background with a really good transducer.
All of them are fairly flat in frequency response, therefore many people don't like Etymotics because they don't have "enough bass". I've found their bass to be awesome, because I like "accurate" over "lively" or "pleasant".