I'm pretty sure anything I'd suggest would already be included in other people's lists so I'm going to request something different. Personally, I'd like a well maintained set of applications that integrates nicely into the default desktop. So things like global shortcuts (ideally having them be the same for common actions), panel and notification integrations, nicely setup default launcher icons, works well with the search bar, looks great with the default icon set, etc.
Example: whatever calendar app you include should work with the calendar in GNOME's panel.
I'd also like to emphasize on the well maintained part since I personally prefer that my core set of applications to not become stale over time or even worse, just have tons of quality of life/paper cut bugs that remain unpatched for years. Whatever is chosen as default should get continued support and help from Ubuntu itself.
If you can do that, I would definitely come back and give Ubuntu another chance.
Example: whatever calendar app you include should work with the calendar in GNOME's panel.
I'd also like to emphasize on the well maintained part since I personally prefer that my core set of applications to not become stale over time or even worse, just have tons of quality of life/paper cut bugs that remain unpatched for years. Whatever is chosen as default should get continued support and help from Ubuntu itself.
If you can do that, I would definitely come back and give Ubuntu another chance.