Best way to compare value in register with constant assembly AVR
The instruction you need has the mnemonic CPI (compare with immediate).

This will set or clear a flag depending on the result.

You can then branch after checking the flag.

Have a look at the following example.  It's from the [8-bit] AVR instruction summary (http://www.atmel.com/images/doc0856.pdf).  

The example below is from page 24:

Example:

cpi r20,5 ; Compare r20 to the value 5
brbc 1,noteq ; Branch if Zero Flag cleared
...
noteq:nop ; Branch destination (do nothing)


There are other instructions that will do slightly different comparisons (like compare two registers, instead of comparing a register with a literal).