In no particular order except for the first one

Junko Enoshima: With the addition of DR3 (Maybe it was elsewhere first but like I've only really played the main 2 games and seen dr3) but Junko is rediculously smart, like outrageously smart. Back in DR1 I saw her more like Trump, just a crazy rich person who can influence people, but in the despair arc it becomes apparent the extent Junko has control over things. For example in her first meeting with Izuru it appears like she simply is crazy by her and her sisters failed attempts to deal with him, and the fact that she's underfoot, but as commented on, Junko planned the whole encounter that way to get what she wanted. This and other things leads me to put Junko at the forefront of the list

2-10

Sonia: lots of literary experience and languages and culture, tons of studying Chihiro: Programming is a very complex task, but Chihiro makes it look easy Kyoko: Reasoning and Detective skills Hopeman the legend: While he at first just appears to be a complete nut, his knowledge far exceeds a lot of the other cast Mukuro: While not as smart as her sister mukuro is incredibly smart and quick witted Togami: Management of that kind of money isn't for stupid people Munakata: Almost heads one of the last organizations Izuru: His abilities give him the spot Yasuhiro: Okay okay hear me out. Guess who was the only one smart enough to stay OUTSIDE during Naegi's wild visit