Unreal 4 right now is the cutting edge for any kind of level design work I have seen. Cryengine is a close second but fails on community support and intuitiveness imo. In this day and age I am going to have to say that source is looking pretty last gen right now on a lot of fronts.
I also have to say that learning a new engine isn't really as bad as it may seem. If you understand the ins and outs of something like source you have already learned a great deal about how all 3d engines work. Chance are a lot of you would pick up UE4 really quickly if you watched over someones shoulder while they worked on something in less than a week.