YouTube: Metal Blade Records / Jo Hale/Redferns, Getty Images / YouTube: Napalm Records

Here are our picks for the five heaviest progressive metal bands ever!

Over the last 40 years, countless progressive metal bands have emerged that - in a simplistic sense - merge the complex compositional structures, lengthy song durations and vibrant instrumentation of '70s prog rock with the unapologetic aggression of multiple metal styles. The genre has gotten perpetually fiercer in recent years, too, leading us to ask: Who are the five heaviest prog metal bands of all time?

So, read on to see which five acts we think do the most justice to the term, and let us know if you'd swap out any of our picks with another hellishly sophisticated ensemble.