Look at the Corp. Office, it is very big, and doesn't lag. It is so big that the fog covers any part of the building just about anywhere on the map, thats probably one reason it doesn't lag as you'd expect. The trick would have to be no not let it be an open map, and make it seem like it is bigger then it really is. The problem is that it takes a long time to download....
As for my favorites, I like terrain maps mostly. Interior maps don't inspire as much creativity, and strictly define were you can reasonably build. Flat maps are nice for builds, but there is nothing to work with map wise, and are more of like changing a template then giving new inspirations to build.
I like terrain maps the best. I think it is a shame that the only default terrain map is the slopes. What I really like about terrain maps is the ability to build in and around things. I actually rather build something creative on a slope then something normal flat. I also like the ability to build into the terrain, opposed to not allowing anything to intersect with pieces of the map.
I think the maps that really score are those that combine things well. An example is ice pack.
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