Whats the best resource to learn Assembly language for PIC microcontroller's
There is more that one PIC architecture, with significantly different instruction sets. Microchip do not have the architectural consistency of Atmel AVR for example. So you need to specify what you are using - PIC12, 16, 24, 32 for example.
So first of all I would suggest avoiding PIC assembler on the basis that what you learn on say a PIC12, that may not be very applicable to a PIC24. Secondly I would avoid using a PIC at all if I possibly could as a software developer - though I concede there are other considerations, and you may have no choice.
What you may have a choice over is not using assembler. While the lower end PICs are not perhaps best suited to C code generation, that's the compiler writer's problem; it is still more cost effective in terms of development time to use C where possible. On the other hand for high volume products, if you can fit the code into a smaller part by using assembler, that may be a factor. Deciding you need assembler before you have tested a C solution is often a 'premature optimisation'. Even if the C implementation fails size or time constraints, you can just call it a prototype, and recode it to meet constraints. Doing than may still be faster than starting assembler coding from scratch and struggling with both design and instruction set at the same time. Tell your boss you'll prototype it in C, then when it meets requirements, no one will want to waste money re-coding it is sone non-portable, unmaintainable assembler code.
Finally to answer your question, assuming you need to get work done quickly and efficiently, use Microchip's examples and app-notes as much as possible, and familiarise yourself with the instruction set from the manufacturers instruction set reference. The instruction set for the lower-end parts is not large. For day-to-day work I like to use an "Instruction set reference card" as an aide-mémoire - it summarises all the essential details of each instruction, usually in a couple of pages - print it double sided and laminate it! ;). Here is a PIC16 example
I would try this: Pic Tutorial
It might be quite a late reply, but you could benefit from this lab course written primarily for learning PIC Assembly through applications.
Check: http://embedded-ju.ucoz.com/
Check the experiments tab, it includes:
Experiment 0 - (Introduction to MPLAB)
Experiment 1 - (Introduction to PIC Assembly Instruction Set)
Experiment 2 - (More on Instruction Set and Modular Programming Techniques)
Experiment 3 - (Basic System Analysis and Design)
3.1. Guide to Hardware I
Experiment 4 - ( LCD - HD44780 )
Experiment 5 - (Keypad)
Experiment 6 - (Using HI-TECH C Compiler in MPLAB) 6.1. PIC Programming with ICD2 Guide.
Experiment 7 - (Timers (Timer0 and Timer2))
Experiment 8 - (USART)
Experiment 9 - (Software PWM & A/D)
It is based on the PIC 16 series, most notably, 16f84A, 16F877A and 16F917 .. the examples are fully commented, and the experiments fully explained.