Generally, speakers with large cones would give you a very good bass punch. These speakers have tweeters installed on them. They are called 2-way or 3-way speakers. I prefer such speakers - Round and about 10 inches in diameter.
You can also opt for Oval speakers which come in 6 x 9 inches sizes. Oval speakers have a inferior frequency response compared to the large round ones.
Pay attention to the magnet, the larger the magnet, better the punch.
Install the speakers on the loft behind the backseats, or, if you plan to door-mount them, make sure the magnet doesn't come in contact with the door. The speakers need sufficient free air in an enclosure to output a good bass kick.
Last but not the least, buy that what sounds good to your ears.
Head units play the second most important role, if you don't plan to install a power amplifier, make sure the head unit can output about 60 W - 70 W RMS.