I use WEED OUT or KILEX, depends which one the store has at treatment time. I have hundreds of those beautiful yellow flowers that seed on both sides of me (thanks nabours) and it works great and for the rest of the time I use scotts WEED and Feed. I only get the odd one now in the lawn but when I bought this house there were hundreds of them.
The trouble with Weed-N-Feed or any similar weed killer/fertilizer combination is that very little of the weed killer gets to the dandelion tap root to completely kill the plant. If you have flowers and ornamental trees in you yard, they will get a high dose of the Weed-N-Feed, and suffer as a result. Also children and pets will be exposed to a lawn spread over by chemicals.
Spot spraying with any non-selective herbicide like Roundup will not only kill the dandelion, but the grass as well, leaving you with dead spots all over your lawn.
Spot spraying with a weed killer designated for lawn use like Weed-B-Gon kills the top foliage, but the tap root usually survives, with a new healthy dandelion back in a few weeks.
So the solution I have found that works is a two step process which involves using both hands for a one-two punch.
In your right hand hold a "weed digger" (that fork shaped hand tool) to lift the dandelion plant and about 1/2 inch of the top of the tap root out of the ground.
In your left hand hold a plastic sprayer filled with Weed-B-Gon. Spray a little (less than a teaspoon) into that small hole that you just lifted the dandelion plant out of. All the weed killer is directed right at the tap root to kill it.
Yes, you'll also have to later go around and pick up the pulled dandelions, but the effect on your lawn will be immediate, before those yellow flowers have turned into the seed machines that will spread throughout your lawn.
An added benefit is that you've used a minimum amount of chemicals on the lawn.