What's the best method for generating 5V at 2A?
Almost any motor that 'self excites' can be used as an alternator or generator - but some are far better choices. You could use a motor or purpose built alternator that needs external excitation to start, but there are enough choices that there is no need to add this complexity. eg a automotive alternator would work well enough but the excitation need is "annoying".

Best are permanent magnet brushless DC motors or their equivalent brushless alternators. Many modern appliances employ brushless DC motors of considerable power ratings and may can be employed as multi phase alternators with minimal modification.

Stepper motors can be used in this way, wit energy capability being roughly related to volumetric size. However, these tend to be salient of "coggy" and have definite stop positions which make smooth running at low speed difficult.

You do not say what physical size is acceptable. If size need not be too small then superb results can be obtained by using the drum drive motors from brushless DC motor washing machines. Because these are required to produce large torques at low speed when direct driving a washing machine drum they are also very suitable as low speed alternators. A washing-machine BLDC motor  fitted with a simple direct drive handle will allow the motor to be spun fast enough by hand to produce 10 to 20 Watts. Simply rectify the 3 phase (usually) and usilise. 

The Fisher &amp; Paykell Smart Drive washing machine is much loved by alternative energy enthusiasts as a source of motors to be used as alternators for wind turbines , water wheels and turbines and more. They can be used to make make a large but excellent hand crank system.

Search for smart drive - you'll find lots. You will want to end up with something smaller, but an F&amp;P SD will get you going wonderfully.