A long time ago (before most bikes were water-cooled) I always felt the 750cc machines were the best all-around bikes. You could commute on them or outfit them with saddle bags and a passenger and go touring, plus, they went pretty good and were a lot of fun. These days a 400-500 offers *almost* the same real-world performance but if you want to carry a passenger, I don't think they would work very well.
125-250's work best in an urban environment so it all depends on what you want to do. You could tour with one but I would avoid highways and carrying much stuff.
