I would suggest you pick a very outgoing, "look at me" type puppy. The more quiet, laid back type might be a bit more work to get to enjoy showing or being a well socialized adult.

As far as picking a puppy conformation wise....

If both parents are quality you have a much better chance of getting a nice show prospect. I have seen pups from pet litters that the nicest pup in the litter still wouldn't make a successful show dog or family pet. DO NOT buy a puppy from a pet store or a back yard breeder!

 Look for good heavy bone, a nice level topline. At 8 weeks I like to see lots of rear angulation, a nice high set tail. When looking at the puppy from the front the legs should be straight and the pup should have a nice wide chest. The feet should be fairly large, round and tight.

 Head....

Well this is tough to always tell, but for me I like as much pigment as possible. The blacker the better and the more the better. You want to see a wide deep muzzle with a defined stop. If the pup has no stop at 8 weeks he probably won't have much as an adult. Avoid a long muzzle that is more narrow. The ears can look really big at this age, but check for nice placement, low set ears can give a nice head a more houndy look. Too high and the dog looks surprised all the time! 

Also VERY important........

Be sure that both parents of the puppy have passed the recommended health testing for the Mastiff breed!!!