Honestly, I have to say Sneak Attack, and I'm going to echo what other posters have said. While there are a PLETHORA of powerful, super cool class features (The entire Thundercaller bard archetype gets a standing ovation from me), Sneak Attack is the perfect, simple class feature. It tells you what to do without giving you any finicky math, and encourages teamwork and cooperation to get the most out of your character.
Originally it was Spellstrike, because flavor-wise it seems like what separates the gishes from the beefy wizards and self-buffing fighters. But then I played a Blade Adept Arcanist gestalted with a Swashbuckler.
Now it's Spell Combat, for mechanically allowing you to be an actual sword-and-magic blending warrior rather than a self-buffing fighter in some moments and an armor-wearing debuffer in others.
A weird mention goes to the Swashbuckler's Opportune Parry and Riposte. It's such a fun ability...but the fact that Paizo is so intent on making it "THE SWASH CLASS FEATURE" means that everyone else is locked out of it. Which is only made worse because I'm feeling more like the Swashbuckler never should have been its own class rather than just an archetype of the Fighter, and the general swashbuckling style should be open to far more classes.
My favorite ability, by far, is the Summoner's Aspect ability, and Greater Aspect.
The Eidolon is a customization outsider buddy, but having the ability to slap some Evo points onto yourself is golden. 4 Natural Armor, an extra pair of limbs, wings, +8 racial to some skills, tremorsense, add some natural attacks, or more? And when Greater Aspect kicks in, you can become a g@**!$n lava shark if you want to.
|Darigaaz the Igniter|
|Sir Charles the Bold|
Anumal companion or familiar for me, just something about the role play value of having a creature that you're attached to which us likely also a huge liability ...
And In all honesty if your dm is letting you have a companion in town that isn't a horse or a dog without complaints from locals or the issues of independent feral animals and townsfolk then he's being too open minded