Ambient Lighting - To find a lily pad table lamp that is good for ambient lighting, it should project most of the light sideways and upwards. Typically, this is accomplished with the lampshade shade. Shapes such as the drum and inverted coolie are some of the best ones for ambient lighting purposes.
Task Lighting - If you are looking for a lamp that's excellent for task lighting, then you should look for the one that will direct most of the light downwards. Some of the best ones will even completely block the light from escaping to the sides and up. For the lampshade shape, the bell, coolie, and empire are some of your best choices.
Keep in mind that there are lamps that can do both task and ambient lighting. However, in most cases, such lamps become a "jack of all trades, master of none." Also, they are typically more expensive.