In Citadels, most versions that are currently available for sale also include the Dark City expansion. Since those extra characters are available, what arrangement of characters do you find the most balanced and competitive? Please clarify your response by indicating why you've chosen to add or remove these particular characters to balance gameplay.
I've found the default characters are the most balanced set of characters. While I haven't played with all of the extra characters, some of them significantly change how the game plays. Bruno Faidutti, the creator of Citadels, says he prefers the default characters on the game's Rules FAQ on his website. Quote:
For the extra characters I have played with, I have a few impressions I can share with you.
Tax Collector: I found without the threat of the thief, people started hoarding money. This reduces some of the tension in the game. Since players can't get robbed, they can save to build up to build the most expensive district. Players were also careful to not have money to pay the tax collector after building.
Emperor: Since you can no longer take the crown for yourself, most people gave the crown to the player to their right. This gave them a better choice of cards in the next character draft, while benefiting the player to the right. The Emperor does make it fairer though how the crown is distributed, but it's also the only character ability the benefits someone else in the game.
Alchemist: The ability to get all your gold back you spend is great, but the Alchemist replaces the Merchant. Since the Merchant can tax green properties and the Alchemist can't, this makes green districts the most worthless ones in the deck. They can't be taxed and provide no special benefit. Green also happens to be the most common color in the district deck
Finally while i haven't played with it, the Diplomat seems to be one of the most powerful characters in the game. It will definitely change the dynamic of the game and make it harder to hold onto powerful purple districts. That doesn't mean that any of these characters are bad, and the opportunity to try them out was a bad inclusion in the game. I just think the default set of characters is the most balanced.
I prefer the standard set with the Diplomat.
The Diplomat improves the game in two ways:
You can also use the Navigator, but you should then switch the Magician for the Wizard --- drawing 4 district cards makes you so tasty a Magician target that you de facto don't have the option.
Other than those changes, I find it best to stick with the base set.
Incidentally, in my group the Witch is completely broken. Player 1 almost always chooses the Witch and is very likely to get the role she wants and avoid the Thief.
I like to choose randomly to have a different setup each time. I agree with other commenters that the base set is easier, so I tend to just choose two of the expansion characters randomly before the game. I then put them in with the base set and remove their equivalents (e.g. if the Witch is randomly chosen, remove the Assassin). Each time I play the setup is a little different so it keeps the game quite interesting.
In my play experience, I've found that I'm not a fan of the Witch or the Emperor. The Witch is more powerful than the Assassin, who I think is powerful enough as it is. On the other hand I find the Emperor to be a little weak. The King is a good out if the start player keeps taking Assassin, or Thief if there is no Assassin, but at least then the King puts themselves in the driving seat, whereas the Emperor seems unnecessarily weaker.