PERFECT map? Doesn't exist of course, but I'll try to explain my opinion on how some maps appears to us "more perfect" than other maps.
Decent visuals are an automatically preassumed basic. Fun gameplay and style consistency are probably what makes a perfect map.
Fun gameplay can mean different things for different people. Some people enjoy getting their ass kicked by gigantic enemy armies and trying again and again until their skill improves, they finally win and feel satisfied. For others is dying too frustrating, so they find enjoyable maps that let them furiously run and blast through and provides only a minimal challenge, but only rarely forces them to stop and hide and feel the need to play carefully. For some of us the "fun" means to keep still being surprised and astonished by something, may it be unexpected monster ambushes or stunning and original visual effects or architecture. Or some people are keen on an atmosphere. The gameplay type we like certainly has major influence on our perception of the map being good.
Style consistency is in my opinion what makes the perfection of the map complete. If several maps had decent visuals and the type of gameplay we like, we'd most like the map which had the style unified. I don't mean monotexturing, I mean balance of visuals, difficulty and detail amount thorough the map, fluent progression, culminating and reaching the climax. Such style that makes our eyes and our heart feel that everything is alright. Makes us feel we can predict what we will find behind the next door, although we can't really know at all! Makes us feel we are already familiar with the map. Yes. A twist may happen, but only as a part of a surprise moment. Otherwise, all progression should be smooth and somehow predictable.
Let me state an example: Kassman's MAP20 from CC4. That map isn't great just because it is detailed. From the beginning, you are encountering an amazing and thoroughly unified visual style, while the difficulty is continuously developing from very easy to harder. At about one half (or two thirds) of the map, not only the difficulty, but also the style appears to start continuously changing into some sort of "virtual abstractness" or something. The gameplay is just my thing - offers me to run and gun through without being much blocked and stopped, but I also feel I shouldn't exaggerate it, or I'll die - and I feel it more and more... This map I would describe as "perfect". With his MAP21 it is very similar, and even better considering that it starts with similar style with which the previous map ends, and continues with the trend of change "into abstractness".
These maps are just superb - lupinx-kassman knows how to make a MAP. I just tried to explain on them what in my opinion makes them so good, and that is what I listed at the beginning of my post.