extremist works good too. Fundamentalist atheist could work too
   but for intellectual integrity that requires the atheist in
   question to always be going "back to basics" and saying, "But
   the dictionary says [x] and because the dictionary says this,
   this means that's all that matters". [the dictionary replacing
   Bible or other holy writ in that case]. But radical is actually
   not a bad word. It's also Progressive and Far Reaching... it's
   Thorough... has wide aims and lofty goals... So really, if *I*
   was atheist (I'm not) and somebody called _me_ a radical
   atheist, I'd embrace the hell out of it and say, "Yeah, I am. Go
   on."   The current marketed image of the atheist by atheists is
   the nebbish bookworm who is intellectual and smart and has
   something to say to the bloody horrible religious world and is
   an innocent in a world of brute savages. But then you had the
   New Atheist movement which kinds wrecked that idea. So, might as
   well run with it if it's whatcha got tongue emoticon