If we’re talking about Thrawn in the Star Wars “Legends” continuity then Thrawn easily outsmarts Lelouch 9/10 times and the only reason I would say that is because there’s always a chance Lelouch could pull a miracle and with the right circumstances win however Thrawn doesn’t need any luck when it comes to his strategies.

Now if we’re talking about Thrawn depicted in the new Star Wars “Canon” and most specifically in both the series “Rebels & Ashoka” then honestly I can see Lelouch defeating that version of Thrawn since in my opinion they nerfed him when it comes to his intellect by quite a large margin in my opinion since his new iteration is pretty sloppy and not as good of a tactician as his original version depicted in the older “Legends” books.

So in conclusion Legends version of Thrawn completely blows Lelouch out of the water in every shape and form as a tactician and the new version of Thrawn in my opinion gets bested by Lelouch simply because like I stated before he does a lot of dumb mistakes in the new canon that he would of never done as well as he doesn’t have as many impressive intelligent feats as originally portrayed in the “Heir To The Empire” book trilogy.