For Mac/iOS –
Fantastical (elegant, freemium),
BusyCal (powerful features, premium),
Cron by Notion (invite only, Mac only),
Calendars by Readdle,
Apple Calendar, Microsoft Outlook,
Calendar 366 (Underrated, Menubar/floating only),
SuperPowered (Menubar only).
Browser-based –
Google Calendar (Most reliable, anywhere)