I've been working on an arcane build for a melee companion/familiar. I wanted to share what I have and listen to your thoughts on other tricks/feats/spells that you can think of that supports the build (no summoners, too obvious, plus I've already played one recently).
Class choices:
Witch Beast-bonded:
was what inspired the idea, but I am sad that their spell list doesn't quite support it very well (plus witches are bad candidates for putting their familiars in harms way). I would take the Transformation tree focusing on beast/dragon shaping the familiar. A breath weapon build best supports this class, giving your familiar flyby attack and shooting breath weapons, but it's too bad the only witch that gets beast shape 4 (the really good one) is the animal tree that loses the other beast shapes and dragon shape. Healing and ward hexes will be important I think.
Sorcerer:
Less fragile familiars and has access to animal companions which have way better natural numbers (aside from the DR and SR that the familiars can get without spells). Access to all the great buff spells, and all the great other spells hehe. Less skills that witch or magus (due to stacking CHA instead of INT) which would be important for a familiar not the animal companion.
Wizard:
leaning more and more away from wizard as compared to the sorcerer.
Magus:
better familiar base attack and great spell list selection, but the better base attack doesn't compete with the higher spell selection of the 1-9 casters. Accessing beast shape 4 that much sooner makes me lean further away from Magus. But I am still exploring magus synergy.
Animal companion would be best choice, Constrictor snake will contend for one of the best polymorphed STR at 19 (not counting STR from leveling but it's 23 minus 4 since it's size large). I think the rhino had the same if not +1 better? I'm still reading up on the newer animal companions. Plus animal companions get more feats, so I'm leaning more toward using one over the familiar.
Familiars for Melee
the best polymorphed familiars as far as STR goes, Earth elemental, Cacoda Daemon, Cythnig Qlippoth all tie in first with a 16 base. The earth elemental would fall behind for it's hideous DEX if it weren't for the fact that its immune to crits and precision damage. The Arbiter Inevitable is only behind by 1 STR but has regeneration and can wield short swords proficiently (gogo magic weapons). The celestial animals are great too but typically have low strength scores (i think a polymorphed fox is the highest at 13?)
Familiars for Breath weapon (beast bonded witch only)
Cant beat the Faerie dragon and the Azata Lyrakien. With the human racial sub "Eye for Talent" they both have high enough CHA to take Eldricht Heritage feat-lines, including the amazing Marid bloodline that lets them shoot high damage water streams at will. A bunch of others can take the first and second stage of Eldricht Heritage but those are the only two I know of that can take all 3.
Human is my race of choice, just for the amazing "eye for talent." I'd be interested in other ideas for this kind of build.
Evolved Familiar gives some nice options.