Best cut.

[jakesi]

What is the best part of meat on a lamb.

[chijiki]

The best part for what?
There is no part of a lamb that cannot be used to make a dish that will be the 'best' for someone.
For roasting most people would choose the leg, shoulder or a rolled breast for example.
For chops to grill or fry most would choose loin, shoulder or leg.
Stewing ..... shank
Soups, stews & some wonderful greek recipes --- head, lungs, intestines, heart & liver.
Lambs Fries, also called Rocky Mountain Oysters, Turkey Nuts, Farmyard Jewels (& a dozen other names) are a wonderful treat using the testicles.
I love sweetbreads .... the pancreas from the stomach or the thymus gland from the neck.
Brains are very nice.
Kidneys are a treat when sauteed gently.
So, I refer you to my opening question - 'best for what'?

[jakesi]

For roasting but I think you have covered everything.

[darth vader]

leg, imho

[incitatus]

or a rack roast/crown roast.

[Janetex]

ewe, I think I feel sick, I don't like lamb much.