Edit: A lot of folks have already complained about Dwight not being in Slytherin, so I thought I would share my reasoning.
While Dwight is ambitious to be sure, he wouldn’t do anything to get ahead. One example is the episode when he has a chance to defeat Pam when she is lying about the new building but then he overhears her fears of being a failure and decides to share the knowledge of how to beat him with her.
He puts on a big front, but he actually does many selfless things throughout the show, he has a wealth of strange knowledge, he is constantly striving to better himself, and he’s seriously eccentric.
I knew this would be a bit of a Hatstall, and I agree he certainly can be seen as Slytherin. In the end I went with his zealous thirst for knowledge over his ambition, since he has never faltered in that attribute.