RJ: Cape Kidnappers is quite walkable now. When we started, I wasn't sure. I played 15 holes with the boys just as we were starting construction, and it was a tough walk -- but we had to climb down through the canyons on #6 and #14. With the bridges, it's much, much easier.
I have yet to design a course that really isn't walkable; I think walkability is important to golf, and not just because of rankings. Black Forest and Lost Dunes have some very long transitions, and Stone Eagle is very hilly, so from a practical standpoint not many people walk the whole course in those cases, but I have walked all three without getting tired enough for it to affect my game. (But, I was younger then.)
Then again, I am not saying I would NEVER build a course that couldn't be walked. There are plenty of sites out there on which that would be impossible, and I would not rule out all of them as unworthy. Even if a cart is involved, most courses provide plenty of opportunity for exercise along the way ... at Stone Eagle, for example, it's common for people to walk most of the holes but hop into the cart with a forecaddie a few times a round, and that works just fine.
I won't list my top 5 because I don't know where you would draw the line of "unwalkable". I played Forest Highlands years ago with Weiskopf and we both walked it ... I carried my bag ... it was a long day but a fun one.