I was looking for the same thing today. As mentioned before, most mainstream modern models should work fine but keep an eye for the ones with switchable graphics especially Sony VAIOs(C series in my case), I face some problems with it from time to time especially when connecting another monitor.
Another thing to look for is to check if the CPU supports virtualization as that would be very useful in enhancing the experience while running Windows(and others) from within a VM right from your Linux box.
One nice brand I came across is one known as
System 76, there guys sell their own brand of laptops and desktops with Ubuntu pre-installed and guaranteed to work out of the box and have the best performance on their hardware. Also their prices are a bit cheaper, their designs are nice and very neat and simple. But most probably they don't exist here.
Other than that, I'd recommend Lennovo think pads. they're powerful, durable and most of them support Ubuntu totally out of the box based on what I've read.
