I started fapping for the release and gather imaginary party setups, so I'm also trying to list stackable bonuses for melee. Here's what I remember from Kingmaker and a few new bits from the wiki:
Freebooter : +5 from freebooter's bane (move action) , +2 when flanking from freeboter's bond (standard action). Dipping only 5 levels gives you +2 from bane and still +2 from bond.
Bard/Sensei/Martyr : +4 morale bonus from Inspire Competence. Dipping only 5 levels still gives you +2, along with mirror image if bard
Cleric : Community domain is incredible , but I don't think any of the npc's can pick it. Good domain's bonus lasts just for 1 round. All are sacred bonuses
Judge : This inquisitor subclass can share its +5 sacred bonus to attacks, AC and saves with the entire party (+2 at L5, +3 at 10 etc) , along with partywide ignore concealment at lvl12. If the base class wasn't so shitty, this too would've been icredible.
Paladin : Mark of justice (lvl11) gives paladin's Charisma bonus to attack rolls as sacred bonus. Too bad its too deep to dip.
Rogue : 4 level dip to get debilitating strike (-2 to enemy AC). Gives a lot of goodies aswell (dex to attacks and damage, various kit abilities etc).
So what else do we have?