Duergar or Myconids.

[ManOWar2]

I'm now in the underdark and I was thinking siding with the duergar, but they are evil and my party is not. Wich is best option.

[Tingly]

You have the chance to make friends with mushroom people, and you have to think about it? SMH my head.

[wendigo211]

Myconids are Lawful Neutral. Duergar are Lawful Evil. If you're looking for a metagaming reason, IIRC there are more quests and merchants if you side with the Myconids than the Duergar.

[Doomvora]

If you are evil, it's best to side with glut or gekh for roleplays sake.
If you're good the good option is to side with spaw.

But it boils down to how you want to play and what you think your party should do based on how you're portraying them.

[Ackranome]

Mushrooms are neutral - Dwarves are evil.

[Agony_Aunt]

Depends on how much of a fan of mushroom soup you are.