The 4 I was thinking of:
Search, Maps, Gmail, YouTube
Chrome is arguably a monopoly if you segment desktop web traffic, but iOS Safari is such a large percentage that Safari can't be ignored.
People will disagree with email - but I challenge you to go look at email threads you're on and see what % of the emails are @gmail.com - not to mention G Suite addresses you don't know are Gmail. Outside of enterprise, there's no alternative.
Not bad either that Android and Chrome are duopolies at this point.