There is no simple answer to that. The scientifically correct answer is buttercups, although the common English (and scientifically incorrect) answer is sunflowers.

The family that has the truest petals is the buttercup family, the Ranunculaceae. Those flowers can vary in the number of petals, even within a species, but ten-twelve is a common number. However, in some of the more primitive flowering plants, the level of leaves just underneath the petals (the "sepals") are not fully distinguished from the petals (both are modified leaves). Some of these plants, such as the magnolia family, the Magnoliaceae, can have many more "petals" than the buttercups. Finally, there is the sunflower family, the Asteraceae (which includes daisies, thistles, etc.); their "petals" are actually miniature flowers that surround a large, central disk, which is in turn made up of a different kind of flower. Individual flowers of this family actually have only five petals, which are frequently fused into a single unit, but a single head of these flowers can itself contain hundreds or thousands of these individual, five-petaled flowers.

It is also of note that the sepals of the monocots (grasses, palms, lilies, and their relatives) are generally brightly colored and thus appear to be "petals", but they're not. If you look carefully, you will find that three of the brightly-colored leaves are above the other three; the three above are the true petals and the three below are the sepals. Look at an iris for one good example.