I think "Best" is something that nobody can answer. If this is for commercial production, then you really have to dive into what features are already provided vs. what will have you come up with on your own. If this is a garage game of you and your buddies will flesh out and you are serious about doing it, your best bet is…
XSI Mod Tool has plugins for the Crytek and Unreal engines. From what I have heard, working with the source engine can be a real pain. From what little I did, it wasn't the most appealing from a coding perspective.
this is more of a for fun, "see if we can do it" project. the group is all students, and we just thought it'd be cool to try it out. My reasoning behind choosing the construction set to Morrowind/oblivion is because there are a lot of mod's already created for oblivion so I didn't think it would be too difficult to figure…