5K pace with longer rests gets my vote, esp. if we are talking 800/1600. Want to keep at least this fast. Really it all boils down to the rest interval- shorter in season and when peaking, longer out when maintaining has been my experience.
You always want to keep the Vo2 max in good shape, IMO. This filters down to all paces. It's the easiest to influence/maintain on the least work input too. 5K pace seems to check a lot of boxes for everyone.