Depends on your defintion of smart. Who is the best mathmetician or engineer. Then i'd probaly go with LEX. Who's the genius at cunning and making schemes to decive people i'd go with Joker. Slade really isn't that
book smart. I mean the guy really never did any feats to show how much brians he got. But he is the most versed in battle techniques and fighting/weaponary skills in the 3. So he's top dog for combat skills.