Note: the skills of each familiar is incomplete been a while since I did the research.

LG+NG=Psudo Dragon with Blur,poison attack
CG=Fairy Dragon, Invis 10 foot radius, Mirror image
LN=Ferret, pick pockets
N=Rabbit
CN=Cat pick pockets, find traps, hide+ silent ( I think the cat is best, why? see tactics below)
LE=Imp poison attack
NE=Dust Mephit blind attack
CE=Quasit

Special Note: Anyone can have a familiar in multiplay method, how?
In multiplay new or old, in old export your chartacter, goto permissions, delete main char, start new mage of desired alignment, make sure he/she has find familiar. memorize, sleep, cast. Goto permissions + reimport your previous char. Have Fun!

Tactics: Because you will loose 1 pt of con+bonus hp if the familiar dies keep them out of combat. Keep them in your pocket until its time for their assignment. The best role for familiars is spy and that is why Cat is best. they can Hide+Move silent at 99% other familiars have lower hide, invis spell( 1 use per day) or no stealth at all. Becarefull I don't recall if traps will kill pets but cats also have a high find traps so if you suspect a trap and no monsters are around check with Cat. Another great tactic is to use an invisable familiar to lead a charge of summoned monsters the badder the better. My Bad ass Cleric/Ranger with cat can summon several fire+earth elementals and let them clean the dungeon out with cat leading the way as he yawns by the door.

Side note: I you wait till ToB you'll get a black cat.