There isn't really such as 'the best laptop' for a pentester to use. It also really depends on the type of pen test you are conducting. We suggest you go with what budget you can afford and opt for as much RAM as possible for running virtual machines (VMs).
What kind of computer do I need for penetration testing
You can use any computer you want for Pen Testing - just go with your preference. Try to opt for what you can afford and feel is right for the task at hand. Where possible use virtual machines for your work, for example, if a different operating system is needed to use a specific tool.
How much RAM do I need for penetration testing
There is no minimum amount of RAM needed, but the more you have the more you'll be able to do simultaneously, especially when it comes to running tools and virtual machines (VMs).