Thats pretty broad.
You can import any model you want. Its best to use them if you have a specific theme you want to make, or just to generally create some uniqueness to your map.
If you want to make a BEST map, well, time and effort will help you accomplish that. Never expect your first map to be your best. The more familiar you get with the editor, the more inginuity will come to you.
-VGsatomi