In my experience, the one fight vs the Gith party was the hardest one, I was really really close to losing.
Anyway, I don't think that all fights need to be picked, we have to use common sense to understand if our party would be able to fight certain enemies.
Really?
The gith fight was the easiest in my experience. I didnt even have sneak cheesing to get it done.
But to be fair I did have level 4s and all in the best gear I could manage.
I would say though the underdark fights are by far harder.
