Apple

iMessage is the reason why apple is such a brand-loyal company and it is why they will never make it cross-platform until some government agency drags them across the finish line kicking and screaming. Love it or hate it (as a longtime iPhone user, I personally think the green/blue bubble discrepancy makes it a bad user experience for both iPhone users AND android), this is the most significant component of apple's walled ecosystem that keeps people from straying away from their garden.