Traits: Reactionary, Maestro of the Society
Feats & Things
1) Master Performer, Human B: Improved Initiative
2) Summoner B: Augment Summoning, Versatile Performance: Oratory
3) Superior Summoning
4) ASI: +1 CHA
5) Versatile Summon Monster (Aerial, Aquatic)
6) Expanded Versatility: Use Magic Device
7) Evolved Summon Monster
8) ASI: +1 CHA
9) Grand Master Performer
Summon multiple leopards/lions/tigers/dire lions/dire tigers (depending on level) as a standard action, sing your song to give them huge bonuses (getting to +7 to hit and damage with the right gear, as a move action).
Lots of spell slots (with the ability to summon even more spells).
Lots of skill ranks with fantastic social skills and knowledge.
Build eidolon as pocket rogue, give it super high Disable Device, Survival, etc.
Crossblooded Accursed Orc Bloodline Mutation Sorcerer + Cyclopean Seer Oracle are by far the strongest gestalt, though not necessarily the best or the most fun.
Sanctified Slayer Ravener Hunter Inquisitor + Ranger covers pretty much all your bases, has numerous offensive buffs, as well as access to Stalwart + Improved Evasion.
If third party stuff is allowed, I've recently fallen in love with a Host of Heroes Aegis + a Living Legend Soulknife. Both of these Medium-esque Occult archetypes give the classes tremendous amounts of versatility, and thematically the classes fit very well together.
I've also played a Spheres of Power Mageknight mixed with a Kirthfinder Rogue that was quite the force to be reckoned with. Spheres in general really gives you access to almost anything you could want, while Kirthfinder provides a lot of power. The upgraded Kirthfinder skills also helped a bunch.
Magus/Stalker. Magus is built around wanting to crit spells and the Stalker is built around crit finishing.
You can pick up maneuvers to help you out when spells can't or just grab boosts to increase your damage. If you go the Archer Magus route, Solar Wind offers a lot of boosts for damage. Sadly you don't get a good Reflex Save, only 3/4ths BAB, and d8 Hit Die.
Aegis/Soulknife. Good all saves, full BAB, 4 + Int skills. The metaforge doesn't do the two classes justice.
Shifter (Spheres of Power) + Striker. The Shifter gets a Path of War archetype that completely destroys its usefulness as a shifter, but it begs for Spheres of Might more than anything else.
I still say one of the most broken gestalt builds is still just a straight up Druid/Cleric.
2 tier 1 full caster classes with some of the most powerful spells in the game, full powered shape shifting, insane levels of healing, all while 100% sharing stat dependency between the 2 classes.
Powerful gestalt builds are all about action economy. It doesn't matter how many spell slots you have if all you're doing is still just one spell per round.
Druid is great, but adding in Cleric doesn't really give you a lot. It's not making you as good as two characters. One exception being if you go Evangelist cleric, for the bard song. Then you're both giving yourself something to do with your move action and buffing your pet.
Also, much as I love Druid, you'd likely be better off playing Razmiran Priest Sorcerer with Sylvan bloodline. More spell slots, broader spell access, and a companion that is easy to raise over level thanks to items that boost bloodline effective levels.
For mundane melee, I have been having fun with this combination:
Viking Fighter/Wild Stalker Ranger.
It has Fighter bonus feats AND Ranger combat style feats, Ranger skills, oh and Barbarian rage/rage powers. Strongest? Probably not, but fun, though.
As for strongest, I don't know, but here are some powerful combinations:
Kensai Magus/Sylvian Stalker Slayer.
Strong assassin build.
Monk of the Four Winds & Sensei Monk/Evangelist Cleric.
Strong party buffer build.
Siegebreaker Fighter/Living Avalanche Brawler.
Strong bowling ball build.
Zen Archer Monk/ Inquisitor.
Strong archer build.
Strong Rogue build.
Strong Fighter build.
Ok. I'm putting this together, slowly, using memory and my phone, so I may be missing something.
This is for an upcoming gestalt 8, mythic 4 game.
Witch (white haired witch/ mirror witch) - fighter (weapon master) 8
1 Power Attack
2 Weapon Focus (beard)
3 Goblin Cleaver
4 Weapon Specialization (beard)
5 Great Cleave
6 Cut from the Air
7 Orc Hewer
8 Combat Reflexes
T1 Meat Shield
T2 Uncanny Grappler
T3 Master Grappler
T4 Maneuver Master
T1 Mythic Paragon
T3 Combat Reflexes
Assuming I made no mistakes, that's:
Cleave all targets medium or smaller within 15' with beard, each hit grabs and throw/)lcrush/swing, one constrict per round
Deflect projectiles with successful aoo (no limit) with beard
Stares at his mirror in the morning to study his spells
I have absolutely no idea if this is viable but it sounds fun :-D
3rd party stuff. A steelfist commando warlord (Dreamscarred Press' Path of War) can provide boosts, BAB and punchingness for a spellcaster who likes touch spells, like an eldritch scrapper sorcerer (probably a debuff-centered bloodline like wildblooded: void-touched), or a wilder (Dreamscarred Press' Psionics).
TL;DR - steelfist commando warlord // wilder (pain surge or raging surge)
Of course ranged damage or effects just makes life easier. You can attack creatures that can't attack you back, you usually make a full attack if you're using weapons. Touch attacks are the most reliable; many things have disturbingly high saves relative to your save DC at low or mid-levels. An armiger (Spheres of Might) makes a surprisingly good TWF gunslinger and an eldritch archer magus adds buffs or spells tacked on to the bullets, without slowing your rate of fire.
TL;DR - armiger // eldritch archer magus
But battlefield control is the ultimate power. An incanter (Spheres of Power) with master of mysteries can do some passive counterspelling, and sphere specialization (divination) duplicates the best wizard school. Various spheres can do battlefield control, e.g. war (requires cooperative allies), nature or time. Those leave them short of feats and squishy. A psychic warrior (Dreamscarred Press' Psionics) fixes both, even if the bonus feats won't be used for SoP magic they can free up the general feats to be exclusively dedicated to that. Also don't underestimate using hustle power to make powers a move action while using spheres with your standard action so as not to burn up power points with both actions.
TL;DR - incanter (master of mysteries & sphere spec: divination replacing bonus feats) // psychic warrior