What is the best arduino board for a robotic arm?
If you are hoping to drive the arm straight from the Arduino, that's not going to work.

What you should do is:


Define the maximum weight and size of the load your arm will carry.
Define expected precision and speed of the movements.
Count the Degrees of Freedom (DoF) your arm will need (1 DoF = 1 simple joint)
Figure out size and materials of each part of the arm
Calculate the torque required at each joint (yes, you need some basic physics calculation)
Identify the motors you want to drive (DC are inexpensive but not very well suited for this, without lots of additional control logic &amp; HW - stepper motors are better, but they still do require some additional effort - servos are the easier way but they are a bit more expensive)
Define the requirements for both the micro controller and the power supply, based on the previous choice.


If you can refine your question, based on the few points I sketched, you will get ahead much faster and with less risk later on to have to backtrack and re-design something.

You might have already answered some of those steps, then I suggest you list here your choices.

Similarly, if you are referring to some link you found (instructables, etc.) please post it.

As reference, hobbyking has an extensive catalog of motors and power supplies. You can find the same items also on ebay, but hobbyking can be used at least as reference for various specs (power, torque, weight, etc.).