Survivability is the top factor in choosing and setting up bots. The best bots in my experience are (in order, ranalds.gift build linked): Ironbreaker Bardin, Mercenary Kruber / Grail Knight, Warrior Priest Saltzpyre / Witch Hunter Captain / Zealot, and Unchained Sienna (with Bomb Balm temp hp talent for ult).
With the above builds, the bots' talents, traits, and properties are focused on survivability/defense.
Ironbreaker is the best bot due to being a tank and has the most defensive buffs out of any career. GK / Merc, WP, and Unchained are mostly for their health generating and defensive ults, talents, and passive abilities. Merc can share "Paced Strikes" (10% increased attack speed) with other players. If you have the GK DLC, you can opt to use GK bot instead of Merc bot. GK with Bretonnian Sword & Shield is very strong and many of his team-wide buffs are very useful to the team.
WP and WHC have very useful support ults and many useful team support passive abilities and talents. Zealot will be much tankier than the WHC, but will not be played optimally by bots (they heal themselves and each other whenever they get to low health). Only switch to Zealot, if you find that the WHC is squishy and you don't have the WP DLC. If playing solo with bots, I prefer taking the Saltzpyre bot's slot either as Saltz or Kerillian, as the other characters' careers are more useful as bots.
Since bots like to block and push, Stamina Recovery is a mandatory Trinket property. If bots run out of Stamina, they will take damage fast. Stamina Recovery affects both the stamina regen speed and the stamina regen delay after pushing. Necklace should have Health and Block Cost Reduction.
I prefer the healshare talent on all of them since Merc and Unchained will generate temp health anyway and bots are not good at utilizing their melee attack temp health talents. They also like healing themselves and each other even if they're not downed so healshare takes advantage of that bot quirk.
If taking grims, Curse Resist is mandatory to improve bot survivability. If not taking grims (or on WP bot), substitute Curse Resist with Revive Speed. This is for better team survivability.
Equip them all with Barkskin for more damage reduction. Bots like face tanking Ratling Gunners and Warpfire Throwers, and they will occasionally dip into Globadier gas. Barkskin will also proc even while downed, so this will give your bots higher effective HP until they are revived.
Since bots like to block, all melee weapons should preferably have Attack Speed and Block Cost Reduction. Charms should be Attack Speed and Power vs Infantry. Faster attack speed also means faster block/push transition before or after doing an attack, helping the bot defensively. Most enemies are of 'infantry' armor type, including all horde units.
Prefer high stamina weapons so bots will have more reserve stamina for pushing and blocking.
I prefer the Opportunist trait since bots like to push and since Opportunist actually adds stagger to all melee attacks, not just pushes. It helps keep the enemies controlled and staggered which is preferable due to the current stagger meta. I didn't consider Swift Slaying since I don't put Crit Chance on the trinket and melee weapon, and wouldn't be able to use it optimally.
Bots use simple chain melee attacks on their melee weapons. They don't seem to know how to block cancel and mix push/light/heavy combos. Equip them with weapons that have simple optimal chain attack patterns: chain light attacks on hordes and chain heavy attacks on elites/monsters, or the reverse - light on elites/monsters, and heavy on hordes.
Bots are good at sniping. I believe they have a 100% bodyshot accuracy. They will try to prioritize pinged units.
I find that bots are more ammo efficient with high ammo count weapons, which is why I prefer the Repeater Handgun on Kruber rather than the Handgun. I also set Bardin to use Drakefire Pistol due to this. It seems that the only ranged weapons that Sienna can use properly are the Fireball Staff and Bolt Staff. Since bots are good snipers, I prefer Bolt Staff. Other staves, she won't be able to use properly. I don't think I ever saw a Sienna bot use the Conflag Staff and Beam Staff properly. She will tickle enemies with the Beam staff but not use the 'explosion on the target' attack, which wastes damage potential. With the Flamestorm staff and Bardin flamethrower Drakegun, bots will just tickle the enemy with light puffs.
I recommend you install the Bot Improvements - Combat mod. Disable the feature: Better Melee Choices, since the weapons I've recommended here work better with it disabled.
Also, install the Loadout Manager mod to automatically set a pre-configured loadout for bots
Spear and Shield (Forgotten Relics DLC) or Sword and Shield - Merc's not as tanky as IB, WP / Zealot, or UC. Shield weapons have higher stamina, less stamina regen delay after pushing (0.75s v 1.5s for other weapons), higher push strength/radius, larger block angle, and lower block cost than non-shield weapons. Shield weapons can block attacks that other weapons cannot, including the Rat Ogre's ground slams. This gives the Mercenary bot more tankiness.
The Spear and Shield's attacks are high-cleave and horizontal, which means that the bot will trigger Paced Strikes often (better with the 'Strike Together' talent to increase attack speed for the whole team).
Dagger - It has a very fast attack speed, and pushes only consume 1/2 shield (bots like to push).
Axe and Shield. The bot will use the light attacks and heavy (bash) attacks optimally with this weapon. It will use the light attacks on elites/bosses and use the heavy (bash) attack on hordes.
Flail and Shield. Has all the advantages of a shield weapon described above. I set Flail and Shield in the 1st slot, and Hammer & Tome in the 2nd slot so that he will always use the Flail and Shield in the 1st slot. I noticed that he will change to the 2nd slot weapon if it's not a Hammer & Tome and not switch back, which is not optimal. The bot likes Flail and Shield's Heavy Attacks which is optimal in most cases since it is a high cleave attack that can also hit armor and damage through shields.
Also, the WP is blocking while charging the heavy attack, which helps in defense. I also notice that he seems to unleash the heavy attacks much quicker than a human can.
Rapier - Fast attack speed and pushes only consume 1/2 shield. Also, lower block cost than most other weapons (only weapons with lower block cost are shielded weapons).