I have a 3-day business trip to Buenos Aires and I am extending by 1 day to see the city.

My flight out is at 11pm that night, so really have a full day to do as much as possible.

Any suggestions?

Sign up for one of the free city walking tours early in the morning and learn about the center of the city, then pick what you like best from that tour and explore a little further. Or ask your tour guide to give you a few more suggestions after the end of the formal tour.

I liked the San Telmo, Retir, and Recoleta neighborhoods best but there are a lot of cool nooks and crannies all over the city.

I'd highly recommend a private tour with Joy of Buenos Aires (she's on tripadvisor and has own website). She has half and full day tours and can customize exactly what you are looking for. We used her for a half day in January 2018 and learned a ton, had a lot of fun, ate well, and got to see a lot of local only spots. Eat dinner at La Carniceria (need reservations).

With time limitation the tour is probably a good idea. If not able then the self walks of the Peron sites and we like the Teatro Colon tour with restaurants nearby are nice.
My fav restaurant is Tantac on Paraguay off Florida.
Advise getting to airport 3 hours before allowing an hour drive there.
Last departure we almost missed the flight with this time allocation. Another story which includes guns drawn at security.

Two words: Don Julio. Great restaurant.