Familiar-nater

[sideswipe]

What feats/class abilities/magic items/anything else would make the toughest familiar that is manageable?

Try and get the familiar at a low ish level. (say by level 10 max)

It has to be a familiar, though i guess animal companion could work if its vastly superior.
But familiar preferred.

As for toughest, that is a loose term. What I mean is it could be tough in the traditional sense, it could be powerful and deal huge damage, have tricks that break the game, buff me to stupid levels or some other powerful thing.

[Vaz]

Improved Familiar to get a Mirror Mephit which then uses componentless Simulacrum to recreate beings by ECL9.

It then makes a Genie, and voila, it can start an infinite wish chain.

[gorfnab]

Familiar Handbook (http://brilliantgameologists.com/boards/index.php?PHPSESSID=v81r0ji74k7diiu61ci3kpeok2&topic=7508.0) -may be of some use

[Urpriest]

In general, if you want your familiar to be a kickass combatant in a traditional sense then you need to be kickass yourself, since its BAB and HD are based on yours. Hexblade and Duskblade both work well with that sort of setup.

I'm partial to Ghostly Visage on a Dread Necro, not for traditional combat but for boosting your fear routine.

[AlltheBooks]

AFB but find a way to turn your familiar into an AC then swap that out for urban companion? Not sure if wild cohort counts as AC? In fact isn't there an ACF that swaps familiar for AC? Why do I post afb? I don't know?

[Gweed]

Bonded Summoner PrC lets you have an Earth Elemental familiar.

The familiar gets larger and tougher as you level.

The requirements are Knowledge Planes 8 and ability to cast lvl2 arcane spells. So you can easily qualify before level 10.

[Falcon X]

Definitely going to have to take a feat (Improved familiar, Divine Familiar, or Dragon Familiar)

Theoretically, the Draconic Familiar or the Earth Elemental are some of the strongest, but they'd have to fit the character.

Same with the Divine Familiars. You have to be pretty Good to get them. But I'm very partial to Coure Eladrin, which can do all sorts of shenanigans, or Musteval Guardinal (Talking rat, at-will Magic Missile).
Take an Imp if you're evil.