The strongest vibrators are electric. They plug into your wall and vibrate with 110 volts of intensity. Battery operated vibrators can make your knees week, but they really can't touch that 110 volt feeling. The Hitachi Magic Wand is widely touted as the most powerful vibrator out there. It can be used for electric clitoral stimulation, or you can get an attachment like the G-Spotter, which allows you to use the vibrator for penetration and G spot thrills.
Rechargeable vibrators are powerful as well. The Iris and the Natural Contours Ideal are great rechargeable choices that will please you over and over. These vibrators combine high power with cordless versatility.
Battery operated vibrators vary in power levels. It depends on the size and number of batteries: A vibrator that runs on four C batteries will be much stronger than one that runs on two AA batteries.
Also take note that a hard plastic vibrator will vibrate more strongly than a rubber vibrator, simply because of the way the two materials diffuse vibration.