Cliffy may be over the top, but I've been a big fan of UT for 14 years now. And the widespread use of the Unreal Engine speaks to its fidelity as well as ease of use. Say what you will about the man, but the results speak for themselves.
"The worst of it all is i used to ENJOY getting excited for big titles like this, but these publishers just keep ruining my love for this wonderful hobby with their endless fuckery." - Sempai