I've had Tracfones for two years now because I have lousy credit, so getting a Cingular or Verizon plan would cost way too much money. Tracfone has great plans, and I've done some research and out of all the prepaid phones, I think they're the best. The only issue I ever had was the quality of the actual phones. These days, you can get really sophisticated ones, with cameras, downloadable polyphonic ringtones, etc. But if you just want a phone that's inexpensive, durable, reliable, easy to use and has a sleek look, get the Motorola C155.
It's only $15! And it comes with ten minutes of airtime. It comes in pink and silver, and it's small enough to look "cool" but not so small that you can't find it in your purse or backpack. My favorite part is that you can customize the menu to put the two options you use most (mine are ringing volume/styles and phonebook) on the main window, and then set up the rest of the menu in order of what you use most.
It also has a color screen, which is almost unheard of for phones under 15 bucks. You can customize the screen with five or six different color schemes and backgrounds, too.
A feature I just realized it has is that even when the phone is turned completely off, the alarm will still ring when you've set it! This is great for conserving battery life at night when you need to use it as an alarm clock.
The only two features I really miss from other phones I've had are a manual volume control/loudspeaker for when you're talking on the phone, and a way to lock the keys to keep them from getting pushed when it's in your purse or pocket. Other than those two things, it's the best cell phone I've ever had by far, and it only cost me $15!