Define "best".
Accuracy, long time stability, resolution, power consumption, ease of use, what?

You can either run the PIC off of an external 4MHz (or whatever you need) x-tal and derive the TMR timebase from the main oscillator. You won't get as slow as 1Hz but you can get, lets say, 100Hz and keep counting interrupts. Or you can use the TMR1 oscillator with a separate (32768Hz ) x-tal. Since it's a 16bit timer it will overflow every 2 seconds if you let it freerun.

/Henrik.