A better question might be.. What do you want to do?

The chips I seem to use the most are:
12F675 or 16F676 for "simple" I/O project that do not need many I/Os.

18F2550 or 18F4550 for bigger projects. These also have on board USB and can be run at high speeds (48MHz) on lower voltages for interfacing with 3.3V parts (SD card).

18F6680 for large projects that do not need USB support.

For learning I would suggest the 18F4550. Has many features that are commonly used and comes in a PDIP package if needed.

But there are many others that are just as good.

In the end the project should decide on the chip.