I have been trialling Busycal and Fantastical and I think Busy cal is winning. It’s group calendaring function enables me to archive all old calendars without having to look at them like in iCal/Calendar.
I think it looks nicer than fantastical, against what many others say here. I think it’s integration with apple’s reminders is excellent.
And I’m using busycontacts to clean up and improve my contacts so hopefully there will be advantages there too.