Opeth kicks all sorts of ass. They are easily the most versatile and diverse metal group there is. Mikael Akerfeldt is am amazing vocalist. His clean voice is so soothing and fragile, but his death growl is incredibly brutal. Yet, you can still understand most of what he sings. If you like Deliverence, check out Blackwater Park. The Drapery Falls is one of the greatest songs of all time.
As for Iced Earth and Meshuggah, well, they're similar in that all three are metal, but that's where it ends. Iced Earth is pretty thrashy, like early Metallica with a Rob halford-esque vocalist. Not overly heavy, not screamy, just plain, loud kick-ass heavy metal. Mushegguah, on the other hand, is just damn brutal. They use 8 and 9 string guitars, detuned to the point of pain, with lots and lots of distortion. It's indescribably heavy, and very difficult for most people to get into, but once you get used to them, and you start to hear all the complexities and polyrythms, you realize just how great the music is, despite being so raw and abrasive.