Investing, stocks, options, pennystocks, undervalued. I'd stay away from wallstreet bets due to all the sarcasm and junk posts.

I believe a straightforward sentiment model from /r/investing would be simple. If you want a challenge /r/wallstreetbets would be harder because the vernacular changes quickly and it’s more context sensitive. /r/news or /r/worldnews if you want to try a “trade the rumor sell the news” sort of classification.