I'll fifth/sixth for the B + M Topline Brake Plus - been using one for about 18months, and I've just bought another for a new bike I'm constructing.

I've also bought a couple of B + M Seculas (one mudguard, and one seatpost) to go complete overkill with the rear lighting. I'll try to get a pic up at later date of them in action.

I've not tried the Seculas yet, but if you're going with only one, and don't mind the rackmounting - B+M Topline Brake Plus is a win.
Do any other dynamo rear lights get brighter when you brake, alerting other road users?