Intel celeron is the lowest processor Intel offers. Below is the list from best to worst:
Intel core i7
Intel core i5
Intel core 2 quad
Intel core 2 duo
Intel core 2 solo
Intel pentium
Intel celeron
For laptops:
Intel core i7
Intel core 2 extreme
Intel core 2 quad
Intel core 2 duo
Intel centrino
Intel pentium
Intel celeron