+++1 on The XS650. Was my 1st bike, 2nd bike, 5th bike, and 7th bike! Yeah it may rattle a bit with with its 360 degree crank, but guess what? You can convert them to a 270 degree and they tick over like a norton! You can bore the hell out of em, slap some Mikuni roundslides on, and other performance engine parts are relatively cheap and accessible (Mike's XS and a few others have almost anything you need). I went from the stock 43hp to about 65 with a 750 kit, megacycle cam, shell thuett heads, 1/2 mile pipes, and mikuni roundslides, and I didn't spend half as much as I did on my cb400f. Plus the low-end torque is really great fun around town. They are really fun to modify, too (although I personally have a severe distaste for "choppers"). If I were to build a brat, that's what I would use.
Yeah, I'd love to have a Norton or Triumph, but lets face it, they are just really expensive around these parts! But two other (not parallel) twins I'd consider would be a BMW 75/5 or 75/6, or an old Guzzi. I has a new Guzzi V11 sport and it was amazingly fun to ride. God I love a bike with 6 gears!