For me personally, writing email faster. It's really easy for me to get immersed trying to write the perfect email or forum post and burn through 40-60min without noticing. They're not even necessarily long, just excessively pruned and reformatted. Getting comfortable with an email with all the important content and okay phrasing saves me a bunch of time.

On second place, priority filtering, i.e. separating email to respond to from subscriptions, offers and notifications. Category filters are nice, but I don't think they're making me more productive.

Anything beyond that is kind of marginal for me. Shortcuts for inserting phrases are fun, but I don't use them that often. If I had to deal with repetitive inquiries, they'd probably be more important.