The "best" is the Testbook T4, but that would require a remortgage, it's what the LR main stealer uses.
Most D2 owners who do have a diagnostics code reader have gone for either Nanocom, Hawkeye or Lynx.
Operationally, they're largely much of a muchness although there are some differences in the area of security. The prices aren't that much different either; a few hundred quid new but you might be lucky and pick up a "pre-loved" device.
The other differences are now probably just in presentation from what I've heard, Nanocom Evolution is a small handheld unit with a colour touchscreen, Hawkeye looks and feels much like other scanners with a navigation button array on the front and the Lynx is an adapter type of unit which plugs into a laptop computer.
The choice is yours.