At the opposite end of blender is possibly sketchup.
A very intuitive and consistent GUI, great community extensions, but quite limited in features to make it very usable as a rendering tool.
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Luckily, simutrans doesn't suffer from this UI problem as badly.
I wouldn't say simutrans is the easiest to learn at first, but at least it is largely consistent.
I guess the only place where there isn't a great deal of consistency is hotkeys between paksets, but I know that is being worked on already.