Just a quick off the top of my head, we reviewed most of the major engines out there at the time we (Sierra & Irrational) started development, including Unreal, Source, NDL's NetImmerse, CryTek, etc. From those we selected the Source engine, but our president at Sierra at the time (Mike Ryder) said we couldn't use anything that had anything to do with Valve, and told us we would use the Unreal engine instead. I believe Unreal was our third or forth choice. To be fair, the engine wasn't bad once we had finished our modifications to it, but it still had some limitations we would have rather avoided.