I love Owl... excellent spot/listen checks, it's nocturnal so it can help stand guard, and it flies! Not bad as familiars go. Ferret/Weasels are nice for style points, and rats are just plain useful.
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For Improved Familiars, I enjoy the services of Psudodragons, with built-in telepathy network, and blindsight to detect invisible nasties. A Sleep poison in a stinger for those close calls (don't forget the save is based on your HD), hide bonuses for size and forest camoflauge, plus flight! A decent package. Elementals are also fine, although Fire and Water Elementals have... limited viability, depending on the campaign. Fire just sets fire to everything it touches, and Water is not so good out of its native element. Earth elementals, on the other hand, are handy for scouting and checking things through solid rock (don't let them get too far away), and Air Elementals are great for arieal scouting, with a 100' speed and high dex scores. All four also have the standard elemental immunities, which is good.
If using the Improved Familiar list from Complete Warrior, I'm partial to Blink Dogs fod their high mobility and built-in blink powers, with the ability to turn the random warping on and off at will as a free action, negating miss chances. It makes for a decent flanker and scout, as well as porting up to 2 characters per jump... more, if your DM is kind and allows the ability to advance after 8 HD. Also comes with low-light vision, Darkvision, and scent, plus Survival as a a class skill. Excellent one you manage to reach +5 BAB/5'th CL... For the more martially-minded, the Winter Wolf is a great companion if you're in the evil-neutral range. Breath weapon every 1d4 rounds, trip, freezing bite, 50' speed, scent (and the +4 bonus for tracking with it), and the Large size means you can ride it too! Of course, the +7 BAB/7'th CL all means that only Gish-types should even try to get this famailiar to ensure the hp so that it isn't a liability.
Animal Companion-wise, I find little beats a Horrid Wolverine.
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