Hah, I've read so many in this genre (one of my favorites!), but I cannot seem to recall any off the top of my head. There have been quite a few good ones, and several slow ones to boot, such fun to read and imagine. I know a few people here have recommended Heinlein, most of his books have been fairly tough reads for me, as they just don't seem to pick up enough momentum for my taste. I've read a few Simak books when I was younger, but they also don't seem to grab me too much nowadays, I guess I've switched over to hard SF that makes plays off of existing technology so it's difficult to really enjoy very old novels (I guess classic SF) when they're tech is so different from the newer stuff or maybe it's their writing styles or maybe something simpler, such as my literary tastes have changed?
Good thing there are so many to chose from, don't have to stick with something that doesn't grab my interest!
