Hrmm. From a quick comparison with a 10th-level fighter, I'd have to say that a 20th-level wizard's familiar is probably equivalent to a 10th-level character. Not as versatile in most ways, but with better AC and SR, though with only about half as many HP as a typical 10th-level character. So, I'd put a familiar's individual CR, separate from its master, at 1/2 its effective hit dice (they have effective HD equal to their master's character level). This may be a bit weak from the familiar's poor Str/natural attacks and HP, but its Spell Resistance and natural armor, plus Improved Evasion, help make up for that, making the familiar tougher. So it'll be almost a paper tiger, with low HP and natural weapon damage, but high natural armor and SR making it kinda hard to successfully wound. Level Adjustment would probably be about equal. These're rough estimates anyway.
