Depends what I want to make.
For anything that takes place mainly indoors, I'd probably base it off some GPLed idtech (3/4) engine or a variant. Good tools, proven tech, lots of free knowledge (and code), close to my personal experience.
Only if large open areas (or crazy physics) were required would I use Cryengine or (preferably) idtech4 with megatextures (ETQW...), or maybe UDK (have no experience with that though).
All hypothetical of course.