Depends on your budget, your taste in sound - as well as your taste in music.
The 'upfront' sound typified by Grado headphones, for example, is great for rock, metal, blues, live recordings, etc. Conversely classical or jazz music may sound 'better' on headphones from Sennheiser, AKG or Beyer.
Headphone amps can be bought for $50 all the way up to $5000. They be portable, plugging into the Line Out of an iPod, for example, or they can run off mains power. They can be solid state or tube-based. However, like many things in audiophile-land, there's a law of diminishing returns once you spend more than about $500.
To get you started, look for names such as
Ray Samuels,
Meier Audio or
Headroom. Meier Audio, for example, have amps with built-in DACs than run off the USB port of your iMac.
Regardless, any 'audiophile' based rig needs (IMO) top-quality input to get top-quality output - especially if you're going as far as to buy a DAC. That means ripping to high bitrate MP3 or AAC or, even better, Apple Lossless.
HTH