Choosing the best lobster to buy is relatively easy: You want a lively one. the livelier the better. It should flap its tail around when pulled out of the water. Dull, dreary, depressed-seeming lobsters have been in the tank a long time. Yes, they are alive, but they won't taste nearly as good as one with the energy to fight back a bit.
You also want one with a hard shell. Lobsters with softer shells have recently molted, or shed their shells. they are likely to have more water and less meat inside.
In any case, keep lobsters alive until you cook them, and do that with a day of buying them.
Also see these guide to How to Cook a Lobster and How to Eat a Lobster.
