>>89960905I'd say that Johns is probably going to go with something similar to what he did before but Brainy is so young here I'm not totally positive. Nevertheless, Johns' stuff isn't a bad start. Technically Lightning Saga is the start but it's a crossover with JLA and not a great story because of it, so just jump ahead to his Superman and the LoSH arc from Action Comics followed by Legion of Three Worlds (people might hate me for telling you to read this here because it's so referential and they're right, but do it anyway because it's still enjoyable without knowing a ton of Legion history and you can reread it later if you decide you like the franchise enough).
Beyond that it mostly depends on what kind of stories you like. Bronze age stuff by Levitz is probably the most popular but you could start around then when Bates comes on. You might need a torrent.